PART II.

DARWIN’S THEORY OF SEXUAL SELECTION.

Darwin seems to have felt the necessity of giving some other explanation for the secondary sexual differences between the male and female than that such differences were only a by-product or concomitant of sex itself. His reason for searching further was probably a part of the general point of view he had reached in regard to the utility of special structures of animals, namely, that their presence finds its explanation on the basis of utility. Believing as he did that most of the adaptations of plants and animals have been built up by the accumulation of small steps, it must have appeared to Darwin inconceivable that the highly developed ornamentation exhibited in the secondary sexual characters could have been simply the by-product of sex itself, especially when the ornamentation may have been entirely absent in males of closely related species. To-day we are not, I think, so oppressed with the difficulties of the situation, for we have become familiar with the fact that very slight genetic differences may cause very great differences in the end-product. In a word, the problem seems less formidable to us than it did to Darwin.

Darwin appealed to three processes to account for the facts: (1) to natural selection between the members of the same sex; (2) to choice on the part of the “other” sex; (3) to the “inheritance of use.” Since each of these appeals to a different procedure, let us take them up separately.

Competition of the males with each other for the female would, Darwin said, lead to the survival of those males best endowed with organs of offense and defense. The spurs of the cock are weapons dangerous for other birds; the horns of the bull and those of deer are used for offense and defense; the mane of the lion is a protection against the teeth of other lions. It is true that these same weapons and shields serve for attack and defense outside the species; but since the female lacks them or has them less developed, they would not seem necessary for survival of the individual against aggression from without. They have developed, then, through competition within the species.

Several objections of greater or less weight have been urged against Darwin’s interpretation. It has been pointed out that the combats within the species are seldom fatal and that the defeated rival finds another mate. If, as a rule, there are as many females as males within the species and monogamy is the rule, all males will find partners sooner or later, all may have offspring, and the offspring have equally good chances of survival. Under these circumstances it is not to be expected that the combat would be likely to lead to the production of males with longer spurs or larger horns.

Darwin realized this difficulty and tried to meet it by another assumption, viz, that the better endowed males would also be more likely to have more offspring. How could this be made probable? Darwin suggested that the strongest males would be in position to mate with the first females to reach maturity, and if these were more likely to have offspring, either because of maternal endowments that made them also more prolific or because the earlier broods would have a better chance of getting food, etc., then the successful competitor would sooner or later impress his advantages on the race.

At other times Darwin suggested that the exceptional vigor that led to the greater development of the character in question would itself be of value and through transmission to the offspring lead to advance in the development of the other character in question. But here the argument shifts to another field of inquiry and survival is ascribed to greater vigor, while the secondary sexual character is carried along in its wake as a sort of correlated effect.

It will be conceded, I think, that such pleading does not help the argument, but exposes rather its inherent weaknesses. There is, however, a line of defense that is permissible. If monogamy is not the rule, if the male captures or attracts several females and keeps a harem, as do the fur seals and walruses, or rules a herd as does the bull, or has a flock as does the cock, or mates more frequently with random females than do some other males, then the advantage of his more developed weapon might lead to more offspring. If it could be shown that such intraspecific weapons prevail more frequently within polygamous species, a fair argument for natural selection might be made. I do not know whether such a census has been taken as yet, but it is true, I think, that in most polygamous groups we find weapons of offense very highly developed. The fur seal has a harem and the male is greater in size, in strength, and in the development of his tusks than is the female. Similarly for the walrus. The bull drives away rival bulls from the herd until through age or injury, or through the development of a better fighter, he is replaced. If the better endowment is due to a genetic factor, we should expect natural selection to keep the race at the highest possible level that variation supplies material for. If, then, we confine the application of natural selection to cases of this sort, the explanation is as valid as is the theory in other fields. Such a conclusion becomes weakened when an attempt is made to apply it to other groups of animals in which it appears improbable that the secondary sexual characters of the male have any obvious value as organs of offense. There are families of beetles, for example, in which the development of the horns of the male are as striking as are those of the ram or the stag. The males of these beetles are not known to fight with each other, nor are they polygamous. It may seem that we must look here for some other explanation, which, if found, might suffice to cover also the case of birds and mammals. In answer to this criticism it may be argued that it is also possible that the other explanation when found need not necessarily apply to the higher animals, where the laws of combat may still give the true explanation. On the whole, I think that, for our present purpose, it will suffice to state it is consistent with the theory of natural selection to accept provisionally this part of Darwin’s theory for those species in the higher groups in which polygamy holds, conceding, however, that even here it may have to be altered when fuller knowledge is gained.

We are more concerned with that special feature of Darwin’s theory of sexual selection that is applied to those cases where the characters are supposed to owe their special development to selection by the individuals of the opposite sex. It is assumed that the female chooses the better endowed males, because of the strong appeal he makes to her sense-organs. Here we must employ perforce or for brevity’s sake the terms used in human psychology, and run the risk at every turn of imputing to other animals the emotions and acquired associations which man himself utilizes. Even granting that other animals possess somewhat similar emotions to ours, there still remains always the danger, in the absence of real evidence, of imputing to them the particular emotion that we call “feeling for beauty”; and the greater danger of imputing an esthetic sense so highly developed that the choice falls in the long run on the suitor better ornamented than his rivals.

OTHER THEORIES TO ACCOUNT FOR SECONDARY SEXUAL CHARACTERS.

Wallace has always been an opponent of Darwin’s theory of sexual selection in so far as it is based on female choice. As already stated, he believes that the difference between the plumage of the male and female in birds is due to natural selection keeping down the ornamentation and high coloration in the female, because these would be expected to expose the female while sitting on the nest to the attacks of enemies, more especially of hawks. In support of this view he points to a long series of species which build exposed nests and in them the female is plainly and inconspicuously colored, while he also points out that in such birds as parrots, toucans, woodpeckers, hangnests, and starlings, which nest in holes or have covered nests, the female is often as highly colored as the male. It can not be denied that he makes out rather a strong case in support of this view, despite the fact that there are other birds, like the Baltimore oriole, that have covered nests and in which the sexes are very markedly different.

Wallace tries to meet cases like the last one by assuming that the covering keeps off the rain; but, if so, why are the sexes still so different? In the case of other highly colored birds, such as jays, magpies, hawks, and crows, Wallace believes that these birds are all aggressive, hence can protect their nests if attacked. As a further support of his view, Wallace points out that in the few cases where the female is more highly colored than the male (as the dotterel, species of phalarope, an Australian creeper) the male incubates the eggs.

Wallace’s suggestion still leaves unexplained the ornamentation of the male, which he tries to account for as the direct result of the greater vitality of the male. He tries to show that excessive ornaments and high coloration develop especially in those parts of the body to which there is an unusual supply of blood or where nerves and blood-vessels emerge to go to the skin or to the muscles.

“If we have found a vera causa for the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation. And, as all the evidence goes to show that, so far as female birds exercise any choice, it is for ‘the most vigorous, defiant, and mettlesome male,’ this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culmination. That culmination will be reached when the excessive length or abundance of the plumes begins to be injurious to the bearer of them; and it may be this check to the further lengthening of the peacock’s train that has led to the broadening of the feathers at the ends, and the consequent production of the magnificent eye-spots which now form its crowning ornament.

“The display of these plumes will result from the same causes which led to their production. Just in proportion as the feathers themselves increased in length and abundance, the skin-muscles which serve to elevate them would increase also; and the nervous development as well as the supply of blood to these parts being at a maximum, the erection of the plumes would become a habit at all periods of nervous or sexual excitement. The display of the plumes, like the existence of the plumes themselves, would be the chief external indication of the maturity and vigor of the male, and would, therefore, be necessarily attractive to the female. We have, thus, no reason for imputing to her any of those esthetic emotions which are excited in us, by the beauty of form, color, and pattern of these plumes; or the still more improbable esthetic tastes, which would cause her to choose her mate on account of minute differences in their forms, colors, or patterns.”

Wallace says, referring to the immense tuft of golden plumage in the best known birds of paradise (Paradisea apoda and P. minor) that springs from a very small area on the side of the breast, that Mr. Frank E. Beddard, who has kindly examined a specimen, says that “this area lies upon the pectoral muscles, and near to the point where the fibers of the muscle converge towards their attachment to the humerus. The plumes arise, therefore, close to the most powerful muscle of the body, and near to where the activities of that muscle would be at a maximum. Furthermore, the area of attachment of the plumes is just above the point where the arteries and nerves for the supply of the pectoral muscles, and neighboring regions, leave the interior of the body. The area of attachment of the plume is, also, as you say in your letter, just above the junction of the coracoid and sternum.” “Ornamental plumes of considerable size rise from the same part in many other species of paradise birds, sometimes extending laterally in front, so as to form breast shields. They also occur in many hummingbirds, and in some sun birds and honey-suckers; and in all these cases there is a wonderful amount of activity and rapid movement, indicating a surplus of vitality, which is able to manifest itself in the development of these accessory plumes.”

There are two serious defects in such an attempt to explain the facts. In the first place, it has been shown in several cases that have been studied that it is not the lessened “vitality” of the female but the suppression caused by the ovary that keeps down the development of the full plumage in that sex. In the second place, the anatomical influences appealed to are imaginary rather than real, for it is by no means apparent that the local exits of blood-vessels and nerves to muscles are at all correlated with the location of the ornamental parts, in the skin. Even when larger blood-vessels run to the region of excessive development of feather ornaments it may well be that they go there because the ornaments in question use them for their nourishment; in other words, Wallace puts the cart before the horse. The top of the head, where crests so often develop, the throat coloration and throat shields of hummingbirds and birds of paradise, the two long tail feathers of several species of hummingbirds, etc., do not arise, so far as known, from regions which are conspicuous for a rich supply of blood and nerves. Wallace’s appeal to underlying organs such as muscles that supposedly influence the special development of the feathers in the skin above does not strike one as a fortunate appeal to physiological principles.

Hudson, in his interesting book, “The Naturalist in La Plata,” has also criticized Darwin’s theory of sexual selection. He has brought together a considerable number of interesting observations that go to show that the displays--dancing, singing, and combats--of males and females have no relation to mating. Many of them involve birds already mated, sometimes several males participating, sometimes males and females together. Some of the tourneys he describes are more elaborate than the mating instincts themselves, yet are not concerned with mating. He attempts to explain them as overflow phenomena, i. e., as expressions of the high vitality of the males, especially at this time. If he is right, then elaborate exhibitions of these kinds have evolved that have no special connection with mating. Are we called upon for a different explanation for other differences that distinguish the sexes? One example will suffice to bring out a curious emotional (?) display that, elaborate as it is, has no apparent connection with mating (p. 269):

“The lapwing display, called by the natives its ‘dance’ or ‘serious dance’--by which they mean square dance--requires three birds for its performance, and is, so far as I know, unique in this respect. The birds are so fond of it that they indulge in it all the year round, and at frequent intervals during the day, also on moonlight nights. If a person watches any two birds for some time--for they live in pairs--he will see another lapwing, one of a neighboring couple, rise up and fly to them, leaving his own mate to guard their chosen ground; and instead of resenting this visit as an unwarranted intrusion on their domain, as they would certainly resent the approach of almost any other bird, they welcome it with notes and signs of pleasure. Advancing to the visitor, they place themselves behind it; then all three, keeping step, begin a rapid march, uttering resonant drumming notes in time with their movements; the notes of the pair behind being emitted in a stream, like a drumroll, while the leader utters loud single notes at regular intervals. The march ceases; the leader elevates his wings and stands erect and motionless, still uttering loud notes; while the other two, with puffed-out plumage and standing exactly abreast, stoop forward and downward until the tips of their beaks touch the ground, and sinking their rythmical voices to a murmur remain for some time in this posture. The performance is then over and the visitor goes back to his own ground and mate, to receive a visitor himself later on.”

Cunningham, who has brought together many interesting cases of secondary sexual differences in his book on “Sexual Dimorphism in the Animal Kingdom,” attempts to show that the development of the secondary sexual characters of the males are due directly to the use of certain parts of the body during courtship--the use of the parts leading to the enlargement and excessive growth of the parts. The effects are believed by him to be inherited, and he tries, furthermore, to show the way in which such acquired characters could be inherited. He makes use of the modern idea of hormones--substances that are elaborated in many organs of the body, whose effects are often most conspicuously produced in other parts of the body. He imagines these hormones to be collected in the germ-cells and transmitted to the next generation, where their presence contributes to the further development of the special region (when it develops) that corresponds to the region in its parent in which the hormone was made. His speculation meets in the first place with the general objections inherent in Lamarck’s theory--objections so well recognized to-day that I need not go over them here. His special appeal to the hormone theory makes use of that theory in a way to which it was never intended to be put, by assuming that an internal secretion formed in one organ can be stored up in another organ, eggs and sperm--an assumption not only unsupported by any evidence, but, as I have stated, one quite foreign to the hormone theory. In fact, Cunningham’s suggestion is nothing more than Darwin’s old idea of pangens, which, being imaginary, could be endowed with all desirable properties. But one can not invoke a chemical substance, even a hormone, and then at the critical moment endow it with special virtues.

A rather unique explanation of the origin of secondary sexual characters is made by Stolzmann. His argument runs as follows: (1) There is a great excess of males in birds; (2) the males left over after mating are useless to the species, since they can not propagate and they consume food needed by the reproducing part of the population; (3) the conspicuous coloration of the male has been evolved in order that he could be seen more readily by birds of prey and the objectionable excess of males removed; the comb of the cock has developed in order that he may be the more easily killed by other cocks.

Stolzmann’s account of the origin of the plumes of the birds of paradise should be immortalized in the literature of the subject:

“Nous comprendrons aussi facilement la présence de longues plumes chez les males de nombreuses espèces, comme p. e. chez les oiseaux de paradis, chez les veuves (Vidua) et chez l’engoulevent africain (Cosmetornis). Telles plumes ont probablement pour but de relantir le vol des males. J’ai constate chez la Loddigesia mirabilis (oiseaumouche péruvien), que le vieux male posséde l’aile quelques millimetres plus courte que le jeune male ou la femelle. Cet avortement des remiges provient assurément a cause de développement extraordinaire de retrices externes chez le vieux male de cet oiseaumouche. Si donc d’une part les retrices allongees rendent le vol plus difficile et d’hautre les ailes plus petites diminuent sa vélocité, le vol du male doit ètre plus lent que celui de la femelle, le poids du corps restant la même. Le développement extraordinaire soit des remiges soit des rectrices, en relantissant le vol des males, rend leur rôle plus difficile, en facilitant en même temps celui des femelles. Nous pouvons prendre comme exemple le Cosmetornis, qui, comme tous les engoulevents, se nourrit d’insectes, qu’il attrape au vol. Chez cet oiseau quelques plumes des ailes se developpent extraordinairement pendant l’époque de reproduction, en retardant visiblement son vol. Il est donc facile a remarquer, qu’alors le male, ayant les mouvements plus lourds, n’est pas en êtat de se procurer la même quantité d’insectes qu’auparavant; ainsi donc la femelle a plus de chances de trouver une nourriture plus abondante.”

Equally worthy of perpetuation is Stolzmann’s explanation of dancing and singing birds:

“Toutes les réunions des males, leurs danses bizarres, leur chant, enfin, ne servent pas probablement a séduire les femelles, mais pour distraire les males, ce qui rend plus faciles les besognes maternelles des femelles et au surplus les protege contre l’assiduite nuisible des célibataires. Darwin lui-mème constate le fait, qu’ordinairement pendant les réunions des males, quand ces derniers sont trop occupes par le combat ou la danse, la femelle s’echappe avec un d’eux pour copuler. Ainsi donc dans ce cas c’est bien la selection naturelle et non la selection sexuelle, qui agit pour la conservation d’équilibre sexuel.”

DISPLAY OF THE MALE.

The antics of male birds at the mating season, their courtship so-called, has played an important rôle in Darwin’s theory of sexual selection. The behavior of many birds at this time is of such a kind as to suggest that the male is exhibiting his plumage before the female for the “purpose” of influencing her choice. The whole paraphernalia of human psychology is imported into the situation and both the consciousness of the male, his intentions so to speak, and the supposed esthetic response or choice of the female is invoked. Even though it be granted that the words that we must make use of, borrowed from human behavior, are such as to imply much more in the direction of consciousness and purpose than is desirable, and that most of the behavior of animals should be stated in a more roundabout and objective way, yet the theory will only work out on the assumption that the female chooses in some sense the more brilliant or ornamental (or effective) male, whether she is “conscious” or unconscious of intention. I doubt if anyone to-day would care to defend seriously the theory on the grounds of consciousness or esthetic value of the exhibition, despite the fact that Darwin’s language often takes this turn and the less-guarded statements of some of his disciples, such as Romanes, show little hesitation in anthropo-morphologizing the entire situation. It is, however, not necessary for the working out of the theory that this complication be introduced into it, for if the female is more likely to mate with a more brilliantly colored than a less brilliantly colored male, the theory may be made to apply regardless of whether she is “conscious” or not of the difference to which she responds.

But there are weighty arguments against such an interpretation of the behavior of the male and female during courtship. In the first place, there is almost no direct evidence to show that the female mates with the more ornamental male. As this is the all-essential requirement of the theory, the almost complete absence of facts in its support leaves the theory resting on a theoretical assumption. It can scarcely pass unnoticed that while there exists a large mass of data describing the secondary sexual characters, there is practically nothing in this accumulation to show that the female makes her selection on differences in coloration or ornamentation. And on the other hand, there is some evidence showing that the female is ready to succumb to the aggressiveness of the male rather than that she “chooses” him.

The behavior of the male under sexual excitement is often described to be of a kind to exhibit before the female his peculiar adornments. That the “purpose” of his exhibition is to show himself off before the female may be conceded, with reservations as to what is meant here by “purpose.” That the male is conscious of the probable results of his conduct is scarcely probable the first time he courted; but that he may have found out the most probable result after the first attempt through “associative memory” is in accord with what the study of “animal behavior” has shown to be possible. In this sense purpose would mean a line of conduct that experience had shown to lead to a certain end. Anticipation or far-sightedness would henceforth characterize such a reaction. Here, however, we venture on very dubious grounds. But the display of the male may be purposeful in a much simpler sense. His activity may be an inborn reflex to visual or other sensory stimuli that is a part of his attack on the female, or possibly a series of reflexes that we may register under the old unanalyzed terms of “desire and fear.” The action calls forth a responsive reflex in the female, for the sexual act is not entirely active on one side, passive on the other, but consists of a series of interreactions on the part of each sex, which, if they pursue a given course, leads to the final mating. The mutual responses appear to follow an automatic course in many cases if the individuals are sexually ready to mate and the environment is propitious. Types of behavior of this kind must be familiar to anyone who has observed domesticated and semi-domesticated animals. The purpose of the display may mean no more than a reaction that leads to a result propitious to the perpetuation of the species if the situation is ripe for such an outcome.

This conclusion still leaves open the question as to whether the display is more likely to be successful, if certain special characters possessed by the species are exhibited. In the absence of any sufficient evidence to show that this is so, and in the light of the very great danger of projecting “our human standards” into the world of other animals, and in view of the fact that related species without such marks are as successful in maintaining themselves, I can not but think that at present we have a good deal to lose in the way of scientific procedure and nothing to gain of scientific value in accepting Darwin’s interpretation of sexual selection based on the display of the male as furnishing an opportunity to the female to make the “best” selection amongst her suitors on the basis of his adornment.

An excellent opportunity to study the problem as to “choosing” by the female is furnished by the mutant races of Drosophila, some of which, differing in a single mutant gene, have wings as different in coloration as black, yellow, or gray, and eyes as differently colored as white, vermilion, or red. Sturtevant put a yellow female with a gray (wild-type) male and a yellow male. The male that first mated was noted and the trio discarded. The female “chose” the gray males 25 times and the yellow only 8 times. In the control combination, where a gray female “chose” between the same two kinds of males, she took the gray male 60 times and the yellow male 12 times. In both cases it “appears” that the female “prefers” the gray male, but this deduction may give an entirely wrong impression as to what is taking place, for the result would be the same in kind if the gray male were more active and mated quicker. This was tested by putting a gray and a yellow female with a gray male and then for control a gray and a yellow female with a yellow male. The result was as follows:

Red ♂ {Gray ♀ 25 Yellow ♂ {Gray ♀ 12 {Yellow ♀ 31 {Yellow ♀ 30

Here the gray male mated slightly oftener with the yellow female than with the other, whereas the yellow male mated much oftener with the yellow female than with the gray one. Both results are explicable on the view that the yellow female, being less active, is more easily captured by the yellow male than is the gray female. This view fits in also with the former experiment, where the yellow male is much less successful than the more active gray male. Such a conclusion gives a more consistent explanation of all the facts than does the theory of female choice, for on the latter we must suppose that the yellow females prefer the gray males and the yellow male prefers the yellow females, etc.

The following results were obtained by Sturtevant when red and white eyed flies were competing:

Red ♂ {Red ♀ 54 Red ♀ {Red ♂ 53 {White ♀ 82 {White ♂ 14

White ♂ {Red ♀ 40 White ♀ {Red ♂ 62 {White ♀ 93 {White ♂ 19

The outcome can be interpreted in the same way as the yellow-gray competition. The red male wins by virtue of his greater activity, while the white female is chosen more often, especially by the white male, because of her passivity (or weaker resistance). It may be claimed that these results do not show that the female does not choose, for such choice, if made, would be swamped by another condition of the experiment, viz, the greater aggressiveness of one kind of male and greater passivity of the other kind of female. This, of course, is true, but the experiment still shows that in these flies other influences are so much greater than “choice” by the female, if it exists, that the postulated effect of the latter practically disappears from the situation.

Mayer’s experiments with the large moth Callosamia promethea furnish important information as to the factors involved in mating. The results are all the more significant from our present point of view because the colors of male and female are in this species markedly different. The wings of the male are black, those of the female reddish brown; the antennæ of the male are large and bushy, those of the female small and slender. Mayer found that the males are attracted by the female from some distance when the latter are put into a glass jar covered by only coarse mosquito-netting, but if the same jars are turned upside down the males are unable to find the female. Females concealed in loose cotton attracted males. Females were put into a box with an open chimney at one end, the other open end being covered by mosquito-netting. A current of air blew into the open end and out of the chimney. The males flew to the end of the chimney from which the air came and fluttered about in the neighborhood. Males are attracted to places where a female has been kept even several hours after her removal. The male finds the female through the sense-organs in his antennæ, for a male whose abdomen has been cut off and the sides of whose thorax are covered with shellac will still fly to the female, but if his antennæ he coated with any substance he no longer seeks the female. If the eyes of the males are blackened they will mate with females “in the normal manner.”

Mayer cut off the wings of females and glued male wings in their places, so that the female looked like a male. Males readily mated with these females. The wings of males were cut off and female wings glued in their place. Mating occurred “with normal frequency, and I was unable to detect that the female displayed any unusual aversion” to such males. Males with female wings pass unnoticed by other males.

In a later paper (1901) Mayer and Soule describe how, when the wings of the male were painted scarlet or green, the males were accepted as readily as normals in competition with them. Experiments were also made by them with the gipsy moth. Wingless males met with more “resistance” from the female than do normal males, but when the eyes were covered the wingless males succeeded as often as the normal males, but the number of observations on which this statement is were far too few to be of any value, and there are several other observations that make any such conclusion from the evidence highly uncertain.

That it is the odor of the females that attracts the male can not be doubted. It might still be claimed that the female chooses amongst her suitors the darkest males, but the evidence gives no grounds for inferring such a choice, and since she will even accept males with female wings when they attempt to mate with her, it does not appear probable that the color of the male is a factor in the result, or at least if it is, then it must be entirely subordinate to the sense of smell in finding the female and of touch after he arrives. There is little or nothing in the behavior of these moths, or in that of the silkworm moth, according to Kellogg, to suggest that vision plays any significant rôle in courtship.

Concerning the genetic situation in insects, there are only a few cases that have been studied. The most instructive are those in which more than a single kind of male exists (two or three), one of which may be like the female, the other quite different. The best worked out cases are Papilio memnon and P. polytes. De Meijere and Punnett have shown from the breeding data that it is possible to frame an explanation of such a sort that the aberrant female differs from the female resembling the male in only a single genetic factor--one not sex-linked (i. e., not carried by an X chromosome), but autosomal. The gene would be of such a sort that it affects the female only--producing no visible effect on the male. Such a conclusion, if established, helps, theoretically at least, toward simplifying the situation in other species, for it shows that genetic factors occur whose influence is on one sex alone; hence the difference between the male and one type of female does in such cases result from a single gene present in both but causing them to be differently colored. There would be no need, then, to assume that the difference had been slowly built up by selection, but rather that the difference arose at some time by a single mutant step. The incorporation of the step in the species would then follow if the effect of the gene were useful in mating or if it had some other primary significance for the welfare of the species, the different effect produced on the male and female being only an unimportant by-product of its action. On the other hand, it should be emphasized that because a single factor difference between the two kinds of females will explain the genetic results, it does not necessarily follow that the difference did arise as a single mutation. The foregoing argument does no more than imply that the difference in question may have arisen in this way, and if so, that the situation, as it exists, would be the more easily comprehended.

In insects and spiders, where dimorphism is as marked as in birds, the mating habits have been studied by a number of naturalists. Here also there are numerous accounts of the display of the male during courtship. The account given by Dr. and Mrs. Peckham are particularly detailed and call for careful consideration on account of their well-recognized accuracy in observational work. Moreover, as a result of their observations, along with those of Montgomery, Petrunkewitsch, and others, we have really fuller information concerning the courtship of spiders than of birds and of mammals.

In the great majority of species where the sexes are different the male is more brightly colored or more ornamental. For example, in a group such as the Attidæ of France, where both sexes are known, the Peckhams state that in 26 cases the male is more conspicuous than the female; in 55 cases the sexes are alike, or if they differ the male is more conspicuous. It appears that in other genera there are cases where the female is more conspicuous than the male. The Peckhams state that possibly as many as 250 species are in this condition. Those females with brighter colors than the males are usually well armed by strong spines. When very young they are like the males and begin to assume the adult form and color when they are a quarter to a third grown. Whether the change depends on changes in the ovary is not known.

The mating behavior of Saitis pulex, a species in which the males and females are much alike, is described by the Peckhams as follows:

“On May 24th we found a mature female and placed her in one of the larger boxes, and the next day we put a male in with her. He saw her as she stood perfectly still, twelve inches away; the glance seemed to excite him and he at once moved toward her; when some four inches from her he stood still and then began the most remarkable performances that an amorous male could offer to an admiring female. She eyed him eagerly, changing her position from time to time so that he might be always in view. He, raising his whole body on one side by straightening out the legs, and lowering it on the other by folding the first two pairs of legs up and under, leaned so far over as to be in danger of losing his balance, which he only maintained by sidling rapidly toward the lowered side. The palpus, too, on this side was turned back to correspond to the direction of the legs nearest it. (Fig. 13.) He moved in a semi-circle for about two inches and then instantly reversed the position of the legs and circled in the opposite direction, gradually approaching nearer and nearer to the female. Now she dashes toward him, while he, raising his first pair of legs, extends them upward and forward as if to hold her off, but withal slowly retreats. Again and again he circles from side to side, she gazing toward him in a softer mood, evidently admiring the grace of his antics. This is repeated until we have counted 111 circles made by the ardent little male. Now he approaches nearer and nearer and when almost within reach whirls madly around and around her, she joining and whirling with him in a giddy maze. Again he falls back and resumes his semi-circular motions, with his body tilted over; she, all excitement, lowers her head and raises her body so that it is almost vertical; both draw nearer; she moves slowly under him, he crawling over her head, and the mating is accomplished.

“After they have paired once, the preliminary courtship is not so long. When this same pair mated a second time, there was no whirling movement, nor did the female lift her body, as at first.” (pp. 37-38).

The courtship of another species, Dendryphantes capitatus, in which the sexes are entirely different, is described as follows:

“The males of capitatus are very quarrelsome, sparring whenever they meet, chasing each other about, and sometimes clinching. It is a very abundant spider with us, so that we often put eight or ten males into a box to see them fight. It seemed cruel sport at first, but it was soon apparent that they were very prudent little fellows, and were fully conscious that ‘he who fights and runs away will live to fight another day.’ In fact, after two weeks of hard fighting we were unable to discover one wounded warrior. When the males are approaching each other, they hold the first legs up in a vertical direction. Sometimes they drop the body on to one side as they jump about each other. These movement are very quick, and they are always ready for a passage at arms. When courting the females they have another movement. They approach her rapidly until within two to five inches, when they stop and extend the first legs directly forward, close to the ground, the legs being slightly curved with the tips turned up. (Fig. 18). Whether it be intentional or not, this position serves admirably to expose the whole of the bronze and white face to the attentive female, who watches him closely from a little distance. (Fig. 19.) The males also give their palpi a circular movement, much as a person does when washing his hands. As he grows more excited, he lies down on one side with his legs still extended. These antics are repeated for a very long time, often for hours, when at last the female, either won by his beauty or worn out by his persistence, accepts his addresses.” (Pp. 45, 46.)

In another species, Dendryphantes elegans, both sexes are brilliantly colored.

“The male is covered with iridescent scales, his general color being green; in the female the coloring is dark, but iridescent, and in certain lights has lovely rosy tints. In the sunlight both shine with the metallic splendor of hummingbirds. The male alone has a superciliary fringe of hairs on either side of his head, his first legs being also longer and more adorned than those of his mate. The female is much larger, and her loveliness is accompanied by an extreme irritability of temper which the male seems to regard as a constant menace to his safety, but his eagerness being great, and his manners devoted and tender, he gradually overcomes her opposition. Her change of mood is only brought about after much patient courting on his part. While from three to five inches distant from her he begins to wave his plumy first legs in a way that reminds one of a wind-mill. She eyes him fiercely and he keeps at a proper distance for a long time. If he comes close she dashes at him and he quickly retreats. Sometimes he becomes bolder and when within an inch, pauses, with the first legs outstretched before him, not raised as is common in other species; the palpi also are held stiffly out in front with the points together. Again she drives him off, and so the play continues. Now the male grows excited as he approaches her, and while still several inches away whirls completely around and around; pausing, he runs closer and begins to make his abdomen quiver as he stands on tip-toe in front of her. Prancing from side to side, he grows bolder and bolder, while she seems less fierce, and yielding to the excitement lifts up her magnificently iridescent abdomen, holding it at one time vertically and at another sideways to him. She no longer rushes at him, but retreats a little as he approaches. At last he comes close to her, lying flat, with his first legs stretched out and quivering. With the tips of his front legs he gently pats her; this seems to arouse the old demon of resistance, and she drives him back. Again and again he pats her with a caressing movement, gradually creeping nearer and nearer, which she now permits without resistance until he crawls over her head to her abdomen, far enough to reach the epigynum with his palpus”. (Pp. 46-47.)

If we lay no emphasis on the implied emotional elements in the behavior of the spiders in this description--terms of emotion borrowed direct from human psychology--there still remain the several types of apparently significant reactions associated with courtship. The statements leave no room for doubt that vision plays an important rôle in the complex reflexes that lead gradually to successful mating. The Peckhams insist that the display of the male is always of a kind to bring before the female the special adornments of the male in whatever part of the body they may lie. The chance of subjective interpretation here is so great that unless the results are carefully checked up by studies of the attitudes assumed by males in species in which the males are without ornament, their interpretation must be taken with the greatest reserve. Assigning, as our authors do, so much by gratuitous implication to the emotional side of the picture prejudices one, perhaps too greatly, against accepting a special (even an implied intentional) exhibition of the specially ornamented parts. On the other hand, if it be conceded that the conspicuousness of the male is an element in the reaction, the very special adornments visible from the front might be supposed to enhance the effect produced in the female. Similar displays of special ornamentation in the male have been described both for birds and insects, but here, too, the question has been raised as to whether such exhibitions are more than an accidental accompaniment of the posturing of the male, for the same kind of behavior is known to occur in other cases where the male is unornamented and resembles the female. Had such a male special ornamentation it would no doubt appear to us that his behavior was “calculated” to display his ornaments.

Dr. and Mrs. Peckham point out that their observations are entirely inconsistent with Wallace’s interpretation of the origin of secondary sexual characters. They find no evidence in favor of his view that the male possesses greater “vital activity.” On the contrary, the female is the more active and pugnacious of the two. They also object to Wallace’s statement of a total absence of any evidence that the female notices the display of the male. In spiders the females “observe” the males with close attention during their courtship. They point out also that, in spiders at least, as the female gradually becomes adult, a male if preferred will have a chance of mating with several females, “and as the mating season lasts for two or three weeks the more brilliant males may easily be selected again and again.” In regard to Wallace’s argument as to the distribution of accessory plumes in humming birds, the Peckhams point out that--

“The pectoral muscles reach their highest development in the hummingbirds, the diurnal birds of prey, and the swallows, and we may, therefore, fairly use these groups to test Mr. Wallace’s explanation of breast plumes. In the swallows and birds of prey we find no such appendages, in spite of their further claim to them, on the ground of great vigor and activity. As to the humming-birds, we find in the genus Aglæactis six species with more or less developed breast-plumes, which are also found in nine other species, scattered through different genera--in all, only fifteen species out of four hundred and twenty-six; while we find in fifty-six species the lengthened and modified tail-feathers, which, according to Mr. Wallace’s view, should be peculiar to the Gallinaceæ.

“Again, there are elongated feathers from the throat or from the side of the neck in thirty-five species, while seventeen have crests from the top of the head, and seventeen, downy puffs from the tarsi.”

From this brief survey of the family we see that, contrary to what we should expect from Mr. Wallace’s theory, although the breast muscles are the seat of the highest activity, breast plumes are the least frequent of all the forms of ornamental plumage.

“We may fairly say, then, that the humming-birds completely refute the proposition that there is any relation between the development of color and accessory plumes and ‘surfaces where muscular and nervous development is considerable.’”

What is true for birds is even more obvious for spiders where the special ornaments are not confined to parts of the body with high muscular development, etc. The writers make the very pertinent criticism that while Wallace objects to assuming the emotional states in females, he is less careful in regard to the males’ emotions when he speaks of the display “under the influence of jealousy or sexual excitement.... The males, in their rivalry with each other, would see what plumes were most effective; and each would endeavor to excel his enemy as far as voluntary exertion would enable him.”

“If the males have so complex an emotion as jealousy, and further, if they are conscious of the value of the plumes, may it not be asked why the female is unable to ‘see what plumes are most effective?’ The mental state in the male is without meaning unless we suppose the female to be affected and pleased.” (Peckham, loc. cit., p. 144.)

In regard to another interpretation of the courtship, the Peckhams point out:

“Mr. Pocock has suggested that the attitude of observant interest on the part of the female spider might be taken to indicate that she was preparing to spring upon her mate and devour him; or that it might simply mean that she was warily guarding herself from his approach. Neither of these suppositions is admissible. In some species the male is not attacked by the female, and when she does wish, as frequently happens, either to avoid or to destroy him, her attitude is totally different. In the former case she turns about and runs rapidly away, or suspends herself by a thread of web. In the second, there is a contraction of all the muscles, the legs are drawn together, and in this crouching position she creeps slowly toward him, as she might if he were a fly, only with something more malignant in her aspect. When she takes this stand the male incontinently flees. When, on the contrary, the female is interested in the male display, she seems perfectly absorbed in watching him, the muscles are all relaxed, unconscious of herself she directs her glance now here, now there, as he moves about; as he continues his mad antics, her appearance gives every indication of pleasurable excitement, and as he comes closer and closer, she yields herself to the impulses which he has awakened in her, and, as in pulex, joins in his dance and whirls around and around as though intoxicated. We claim, then, to have completely answered Mr. Wallace’s first objection.” (Peckham, loc. cit., pp. 145, 146.)

Finally, in regard to the specific character of the display of the males, the Peckhams make the following significant statement:

“The spider has four pairs of legs, and all are equally available for display or locomotion, and since all the movements are slow and on the ground they are entirely open to observation and study, and we are thus in a position to decide by facts whether their activity is simply an outlet for superfluous energy, and therefore meaningless, or whether there is a purpose in it. If the purpose of the antics is only to let off energy, then we should expect one pair to be flourished around quite as often as another, and that the pair flourished should as frequently be one that was not ornamented as one that was; and, moreover, their movements ought not to be of such a nature as to display the color or ornament, more frequently than the law of chance would explain. If the spider almost always moves the ornamented legs, and in such a way, too, as to bring out their beauty, it would seem to us, to say the least, highly improbable that the dance of the spider was merely a meaningless overflow of surplus energy. Such an explanation leaves much that needs explanation. The facts are, that the best foot is put forward; and this is just what Darwin’s theory requires and explains. Under Mr. Wallace’s view the facts are inexplicable. The better to show that these movements are not simply meaningless outlets of high vigor, we illustrate the several positions by figures taken from nature (figs. 7-12). The figures would seem to prove that the legs that are ornamented or contrasted in color are also the legs that are usually flourished; that where none of the legs have special ornament, then all are used; or, as sometimes happens, when an unornamented leg is used the movements are of such a character as to display some ornament that would otherwise have been more or less hidden from the female.” (Peckham, loc. cit., p. 147.)

In the tarantula, Petrunkewitsch finds that sight plays no rôle in mating--that it is due entirely to accidental contact between the male and female. Here the sexes are closely alike, except for a pair of hooks on the front legs of the male, by means of which he grasps the mandibles of the female, holding them during the elaborate process of transference to her genital opening the sperm that he has already collected in the genital spoon on his palpi. The hooks serve to guard the male against injury or death, while at the same time they aid him in the act of coitus.

In a common spider, Mœvia villata, two kinds of males exist. Both have been seen to mate with the same female. No preference is given to either type. The difference between them, according to Painter, is connected with or caused by an additional pair of chromosomes in the gray male. The two types may therefore have no connection with sexual selection, but be directly due to a difference in the chromosome group.

Montgomery, who made observations on the courting habits of several species of spiders, states that his “general theoretical conclusions were quite different from those of the Peckhams.” It turns out, however, that his objection to their view is based entirely on their assumption that the male is conscious of his display and that the female is guided by an esthetic sense in selecting the more beautiful male. It should be pointed out that even after the removal of these gratuitous assumptions as to the cause of the evolution of the male and female, enough still remains in Montgomery’s own observations to include his results on courtship under Darwin’s theory of sexual selection. For example, Montgomery says:

“The adult male is excited simultaneously by fear of and desire for the female, and his courtship motions are for the most part exaggerations of ordinary motions of fear and timidity. By such motions he advertises himself to the female as a male, but there is no proof that he consciously seeks to arouse her eagerness by esthetic display--there seems to be no good reason to hold that the female is actuated in her choice by sensations of beauty.... Thus my opinion was opposed to Darwin’s theory.”

Now, it is obvious that if a more brightly colored male has a better chance of “advertising himself” to the female all the essential requirements of Darwin’s theory are fulfilled, regardless of whether the male is conscious of his ornamentation or the female makes use of an “esthetic sense.” In another passage (p. 173) Montgomery concedes all that any modern critical advocate of Darwin’s theory is likely to ask:

“We have previously seen that conscious aesthetic choice by the female probably does not account for such male characters [secondary sexual characters with their ‘conspicuous color markings’]; that they are accordingly, probably not due to sexual selection. These characters of the males may be most readily explained as being conceived by simple natural selection. Peculiar ornamentation would be selected because unusually greater sex recognition therefore prompted mating.”

It is evident that Montgomery has only shifted the situation, although to advantage, I think, but is essentially in accord with Darwin’s theory of sexual selection, despite his protest to the contrary. The difference lies in Darwin’s and especially in the Peckhams’ use of the term “choice,” “aesthetic sense,” etc., to stand for the fact that the female more promptly mates (as Montgomery prefers to put it) with a male peculiarly ornamental.

The most critical observations on sexual selection that have been made in the group of insects are those by Sturtevant on the pomace fly. The courtship is described as follows:

“The first and most noticeable act in courtship occurs when the male, being near the female, extends one wing at about right angles to his body, and vibrates it for a few seconds. The wing is then returned to the normal position and the process is repeated, usually with the other wing. But between times there is a scissors-like movement of the wings repeated several times. This vibrating of the wings is often repeated many times, and may be done in any position relative to the female, though the male always faces her. Usually, in fact, he swings quickly around her in a semicircle once, or oftener, during the process. Soon the male begins to protrude his genitalia and, if the female remains quiet, to lick her posterior end. Some white matter now protrudes from her ovipositor, and other males in the same vial are usually observed to become excited now and begin courting, indicating odor as a cause of sexual excitement. If the female runs or flies away the male is excited, moves his wings jerkily, and walks around rapidly, but seems unable to follow the female accurately or to locate her quickly. The penis is directed forward by bending up the abdomen underneath, towards the thorax, and is jerked toward the female (the male always standing facing her at this stage), but not always toward her genitalia, as I have seen it strike her in the eye. (The male in this case, however, had white eyes, and so was perhaps blind. Normally the aim is accurate.) If it does strike the mark the male mounts on the female’s back, between her wings. Mounting never takes place until after the actual copulation has occurred, in which respect Drosophila differs from some related flies (e. g., Muscidæ, Anthomyidæ, Sepsidæ, Borboridæ, and Ephydrichæ, so far as my observations go). In these forms the male flies and lights on the female, after which copulation may or may not take place, probably depending upon the way the female responds.”

To test whether the wings have any significance in courtship, the wings of a male were clipped off and he was put into competition with a normal male of the same stock, age, and size. A virgin female sexually mature was given to these two males. The normal male mated 72 times before the other, the clipped male 53 times. It might appear that the female selected the normal male in preference to the clipped one, or possibly that the male with normal wings drove the other male away. That the operation on the wings may have an influence on the male himself is shown in McEwen’s results. He found that clipped males lost their heliotropism. It was also possible that the courtship of the normal male might make the female ready to copulate and then she would mate with either male. Sturtevant tested the last supposition by placing single pairs in vials, testing each day an equal number of normal and clipped males. The length of time before copulation was noted. The clipped male began to court as soon as the normal, but a larger number of normal males mated in the first 12 minutes than clipped males (50 to 25). Had the females discriminated against the clipped males to an equal extent we would have expected a much greater excess than 72 to 53 when they were in competition. It appears, then, that the wings are useful in shortening the time between the meeting of the individuals and copulation. The display acts, however, almost as favorably for the other male as for the exhibitor himself. The results show, therefore, that here an esthetic sense of the female need not be postulated, for she actually shows little preference when she has been brought to the point of mating between the male that aroused her and the other male that did not. This critical test puts the problem in a different relation from that which Darwin’s theory of female choice was meant to throw light upon.

The reverse experiment--a clipped and a normal female of the same age, size, etc.--showed that the mate did not discriminate between them, for in 52 first trials the normal female was paired with 25 times, the clipped 27 times.