Part 2

It is a very general popular belief at the present day that the Darwinian theory is simply no more than a capricious and anti-theological assertion that mankind are the modified descendants of ape-like ancestors.

Though most of my readers, I do not doubt, know how imperfect and erroneous a conception this is, yet I shall not, I think, be wasting time in stating what the Darwinian theory really is. In fact, it is so continuously misrepresented and misunderstood, that no opportunity should be lost of calling attention to its real character. Bit by bit, naturalists had succeeded in discovering the order of nature—so far that all the great facts of the universe, the constitution and movements of the heavenly bodies, the form of our earth, and all the peculiarities of its crust, had been successfully assigned to one set of causes—the properties of matter, which are set forth in what we know by the name of the “laws of physics and chemistry.” Whilst geologists, led by Lyell, had shown that the strata of the earth’s crust and its mountains, rivers, and seas were due to the long-continued operation of the very same general causes—the physico-chemical causes—which at this moment are in operation and are continuing their work of change, yet the living matter on the crust of the earth had to be excluded from the grand uniformity which was elsewhere complete.

The first hypothesis, then, which was present to Mr. Darwin’s mind, as it had been to that of other earlier naturalists, was this: “Have not all the varieties or species of living things (man, of course, included) been produced by the continuous operation of the same set of physico-chemical causes which alone we can discover, and which alone have been proved sufficient to produce everything else?” “If this be so,” Mr. Darwin must have argued (and here it was that he boldly stepped beyond the speculations of Lamarck and adopted the method by which Lyell had triumphantly established Geology as a science), “these causes must still be able to produce new forms, and are doing so wherever they have opportunity.” He had accordingly to bring the matter to the test of observation by seeking for some case of the production of new forms of plants, or of animals, by natural causes at the present day. Such cases he found in the production of new forms or varieties of plants and animals, by breeders. Breeders (the persons who make it their business to produce new varieties of flowers, of pigeons, of sheep, or what not) make use of two fundamental properties of living things in order to accomplish their purpose. These two properties are, firstly, that no two animals or plants, even when born of the same parents, are exactly alike; this is known as Variation: secondly, that an organism, as a rule, inherits, that is to say, is born with the peculiarities of its parents; this is known as Transmission, and is simply dependent on the fact that the offspring of any plant or animal is only a detached portion of the parent—a chip of the old block, as the saying is. The breeder selects from a number of specimens of a plant or animal a variety which comes nearest to the form he wishes to produce. Supposing he wished to produce a race of oxen with short horns, he would select from his herd bulls and cows with the shortest horns, and allow these only to breed; they would transmit their relatively short horns to their offspring, and from these again the cattle with the shortest horns would be selected by the breeder for propagation, and so on through several generations. In the end a very short-horned generation would be obtained, differing greatly in appearance from the cattle with which the breeder started.

Now we know of no facts which forbid us to suppose that could a breeder continue his operations indefinitely for any length of time—say for a few million years—he could convert the short-horned breed into a hornless breed; that he could go on and thicken the tail, could shorten the legs, get rid of the hind limbs altogether by a series of insensible gradations, and convert the race into forms like the Sirenia, or sea-cows. But if he could do this, you have only to give him a longer time still and there is no obstacle remaining to the conversion, by the same kind of process, of a polyp into a worm, or of a worm into a fish, or even of a monkey into a man.

So far we have supposed the interference of a breeder who selects and determines the varieties which shall propagate themselves; so far we have not got a complete explanation, for we must find a substitute in nature for the human selection exercised by the breeder. The question arises, then, “Is there any necessary selective process in nature which could have operated through untold ages, and so have represented the selective action of the breeder, during an immense period of time?” Strangely enough, Mr. Darwin was led to the discovery of such a cause existing necessarily in the mechanical arrangements of nature, by reading the celebrated book of an English clergyman, the Rev. Mr. Malthus, On Population. On happening to read this book, Mr. Darwin himself tells us that the idea of “natural selection” flashed upon him. That idea is as follows. Not only among mankind, but far more largely among other kinds of animals and of plants, the number of offspring produced by every pair is immensely in excess of the available amount of the food appropriate to the particular species in question. Accordingly, there is necessarily a struggle for existence—a struggle among all those born for the possession of the small quantum of food. The result of this struggle is to pick out, or select, a few who survive and propagate the species, whilst the majority perish before reaching maturity. The fact that no two members of a species are alike has already been shown to be the starting-point which enables the breeder to make his selection. So, too, with natural selection in the struggle for existence; the fact that all the young born of one species are not exactly alike—but some larger, some smaller, some lighter, some darker, some short-legged, some big-eyed, some long-tongued, some sharp-toothed, and so on—furnishes the opportunity for a selection. Those varieties which are best fitted to obtain food and to baffle their competitors, gain the food and survive, the rest perish.

We have, then, to note that the hypothesis that there must be a selection—which was framed or deduced as a “test hypothesis” from the earlier hypothesis that species have arisen by the action of causes still competent to produce new forms—led Mr. Darwin to the discovery of this great cause—the “natural selection,” or “survival of the fittest,” in the struggle for existence. Just as the breeder can slowly change the proportions of the animals or plants on which he operates, so in inconceivably long periods of time has this struggling of varieties, and the consequent natural selection of the fittest, led to the production, from shapeless primitive living matter, of all the endless varieties of complicated plants and animals which now people the world. Countless varieties have died out, leaving only their modified descendants to puzzle the ingenuity of the biologist.

Of the tens and hundreds of thousands of intermediate forms we know nothing by direct observation. They have perished as better fitted forms ousted them in the never-ending conflict. But we feel sure that they once were in existence, and can infer what was their structure, and what were their peculiarities, by the study of the structure and attributes of their now living descendants.

If all the forms of life at present living are the modified offspring of a smaller number of ancestral forms which have died out, and if these again were the modified descendants produced by ordinary parentage of a single original living thing—then the whole series of forms that have ever lived could, if we had them before us, be arranged in the form of a great family-tree—the various branches presenting a perfect gradation of forms arranged one after another, leading down from the terminal twigs (which would represent the latest forms produced) to larger and larger branches, until the common trunk representing the original ancestor would be reached. Our actual means of observing the genealogical affinities of different kinds of animals and plants may be understood by a further use of the metaphor of a genealogical tree in shape like an elm or an oak. Suppose the genealogical tree completely written out—a perfect record—to be sunk in muddy water so that only its topmost branches and twigs are here and there visible—then you have a fair notion of the present condition of the great family of organisms. Only the topmost twigs remain visible, the rest of the great family-tree of living beings is hidden from view, submerged beneath the muddy waters of time. Naturalists have, however, undertaken to reconstruct this great genealogical tree. It is a main object now in the study both of zoology and of botany to find out what are the cousinships, what the exact genetic relationships of all the various species of plants and animals, and so to show, even to the minutest detail, in what particular ways physico-chemical causes have brought about and modified the forms of living things.

The task is not quite so difficult as the comparison to a submerged forest-tree would lead one to expect; at the same time it is more difficult than those who have boldly attempted it appear to believe. We have one great help in the carefully worked out systematic classification of animals and plants according to their structure. We are justified in assuming as a general law that animals or plants of like structure have descended from common ancestors—that is to say, that the same kind of organisation (especially where a number of elaborate details of structure are involved) has not been twice produced by natural selection. Thus we are entitled to conclude that all the animals which have a backbone and pharyngeal gill-slits combined—the Vertebrates, as we call them—have descended from a common parent; that all the animals with a muscular foot-like belly and lateral gill filaments, the Molluscs, have also had a common parent, and so on.

A classification according to structure goes then a long way towards mapping out the main lines of the family-tree of organisms. We are further assisted in the task by the fossil remains of extinct organisms which sometimes give to us the actual ancestors of forms now living. But the most remarkable aid to the correct building-up of the pedigree of animals at least (and the remarks which follow are confined to that division of the organic world), is afforded by the changes—the phases of development—which every animal exhibits in passing from the small shapeless egg to the adult condition. The aid which we here obtain depends on the following facts. Just as we suppose any one animal—say a dog—to have developed by slow change through an immense series of ancestors which become simpler and simpler as we recede into the past until we reach a small shapeless lump of living matter devoid of structure, so do we find actually as a matter of fact, which any one can see for himself, that every individual animal begins its individual life as a structureless particle which is thrown off from its parent, and is known as the egg-cell (Fig. 1). Gradually passing through a series of more and more elaborated conditions of structure, that egg grows into the adult dog. The changes which have taken countless ages in the one case, are accomplished in a few weeks in the other.

And now we have to note the important fact which makes this process of development so intensely interesting in relation to the pedigree of the animal kingdom. There is very strong reason to believe that it is a general law of transmission or inheritance, that structural characteristics appear in the growth of a young organism in the order in which those characteristics have been acquired by its ancestors. At first the egg of a dog represents (imperfectly, it is true,) in form and structure the earliest ancestors of the dog; a few days later it has the form and structure of somewhat later ancestors; later still the embryo dog resembles less remote ancestors; until at last it reaches the degree of elaboration proper to its immediate forefathers.

Accordingly the phases of development or growth of the young are a brief recapitulation of the phases of form through which the ancestors of the young creature have passed. In some animals this recapitulation is more, in others it is less complete. Sometimes the changes are hurried through and disguised, but we find here and there in these histories of growth from the egg most valuable assistance in the attempt to reconstruct the genealogical tree. The history of the development of the common frog is a good illustration of the kind of evidence in question.

The frog’s egg first gives rise to a little aquatic creature with external gills and a tail—the tadpole—which gradually loses its gills and its tail and acquires in their place lungs and four legs (Fig. 2), so as now to be fitted for life on dry land. From what we otherwise know of the structure of the frog and the animals to which it is allied, we are justified in concluding that the tadpole is a recapitulative phase of development, and represents to us more or less closely an ancestor of the frog which was provided with gills and tail in the adult state, and possessed neither legs nor lungs.

A less familiar case is that of a certain kind of shrimp, which is illustrated in the wood-cuts (Fig. 3 and right lower corner of Fig. 4). The little creature which issues from the egg of this shrimp is known as the “Nauplius form.” Many animals very different in appearance from this shrimp make their first appearance in the world as Nauplii; and it appears probable that the Nauplius-phase is the recapitulative re-presentation of an ancestor common to all this set of animals, an ancestor which was not exactly like the Nauplius, but not very different from it.

The Nauplius of our shrimp gradually elongates. At first it has but three pair of limbs, but it soon acquires additional pairs, and a jointed body, and thus by gradually adding to its complexity of structure as seen in Figs. 5 and 6, it approximates more and more to the adult form from the egg of which it originated.

And now we are approaching the main point to which I wish to draw the reader’s attention. In attempting to reconstruct the pedigree of the animal kingdom and so to exhibit correctly the genetic relationships of all existing forms of animals, naturalists have hitherto assumed that the process of natural selection and survival of the fittest has invariably acted so as either to improve and elaborate the structure of all the organisms subject to it, or else has left them unchanged, exactly fitted to their conditions, maintained as it were in a state of balance. It has been held that there have been some six or seven great lines of descent—main branches of the pedigree—such as that of the Vertebrates, that of the Molluscs, that of the Insects, that of the Starfish, and so on; and that along each of these lines there has been always and continuously a progress—a change in the direction of greater elaboration.

Each of these great branches of the family-tree is held to be independent—they all branch off nearly simultaneously from the main trunk like the leading branches of an oak. The animal forms constituting the series in each of these branches are supposed to gradually increase in elaboration of structure as we pass upwards from the main trunk of origin and climb further and further towards the youngest, most recent twigs. New organs have, it is supposed, been gradually developed in each series, giving their possessors greater powers, enabling them to cope more successfully with others in that struggle for existence in virtue of which these new organs have been little by little called into being. At the same time here and there along the line of march, certain forms have been supposed to have “fallen out,” to have ceased to improve, and being happily fitted to the conditions of life in which they were long ago existing, have continued down to the present day to exist in the same low, imperfect condition. It is in this way that the lowest forms of animal life at present existing are usually explained, such as the microscopic animalcules, Amœbæ and Infusoria. It is in this way that the lower or more simply-made families of higher groups have been generally regarded. The simpler living Mollusca or shellfish have been supposed necessarily to represent the original forms of the great race of Mollusca. The simpler Vertebrates have been supposed necessarily to represent the original Vertebrates. The simpler Worms have been supposed necessarily to be the stereotyped representatives of very ancient Worms.

That this is, to a certain extent, a true explanation of the existence at the present day of low forms of animals is proved by the fact that we find in very ancient strata fossil remains of animals which differ, ever so little, from particular animals existing at the present day; for instance, the Brachiopods (lamp-shells), Lingula and Terebratula, the King-crabs, and the Pearly Nautilus are found living at the present day, and are also found with no appreciable difference in very ancient strata of the earth’s crust; strata deposited so long ago that most of the forms of life at present inhabiting the earth’s surface had not then been brought into existence, whilst other most strange and varied forms occupied their place, and have now for long ages been extinct.

Whilst we are thus justified by the direct testimony of fossil remains in accounting for some living forms on the hypothesis that their peculiar conditions of life have been such as to maintain them for an immense period of time in statu quo unchanged, we have no reason for applying this hypothesis, and this only, to the explanation of all the more imperfectly organised forms of animal or plant-life.

It is clearly enough possible for a set of forces such as we sum up under the head “natural selection” to so act on the structure of an organism as to produce one of three results, namely these; to keep it in statu quo; to increase the complexity of its structure; or lastly, to diminish the complexity of its structure. We have as possibilities either BALANCE, or ELABORATION, or DEGENERATION.

Owing, as it seems, to the predisposing influence of the systems of classification in ascending series proceeding steadily upwards from the “lower” or simplest forms to the “higher” or more complex forms,—systems which were prevalent before the doctrine of transformism had taken firm root in the minds of naturalists, there has been up to the present day an endeavour to explain every existing form of life on the hypothesis that it has been maintained for long ages in a state of Balance; or else on the hypothesis that it has been Elaborated, and is an advance, an improvement, upon its ancestors. Only one naturalist—Dr. Dohrn, of Naples—has put forward the hypothesis of Degeneration as capable of wide application to the explanation of existing forms of life; and his arguments in favour of a general application of this hypothesis have not, I think, met with the consideration which they merit.

The statement that the hypothesis of Degeneration has not been recognised by naturalists generally as an explanation of animal forms, requires to be corrected by the exception of certain kinds of animals, namely, those that are parasitic or quasi-parasitic. With regard to parasites, naturalists have long recognized what is called retrogressive metamorphosis; and parasitic animals are as a rule admitted to be instances of Degeneration. It is the more remarkable whilst the possibility of a degeneration—a loss of organisation making the descendant far simpler or lower in structure than its ancestor—has been admitted for a few exceptional animals, that the same hypothesis should not have been applied to the explanation of other simple forms of animals. The hypothesis of Degeneration will, I believe, be found to render most valuable service in pointing out the true relationships of animals which are a puzzle and a mystery when we use only and exclusively the hypothesis of Balance, or the hypothesis of Elaboration. It will, as a true scientific hypothesis, help us to discover causes.

We may now examine a few examples of undeniably degenerate animals, and first, I may call to mind the very remarkable series of lizard-like animals which exist in the south of Europe and in other countries, which exhibit in closely related genera a gradual loss of the limbs—a local or limited Degeneration. We have the common Lizard (Lacerta), with five toes on each of its well-grown fore and hind limbs; then we have side by side with this a lizard-like creature, Seps, in which both pairs of limbs have become ridiculously small, and are evidently ceasing to be useful in the way in which those of Lacerta are useful; and lastly, we have Bipes, in which the anterior pair of limbs has altogether vanished, and only a pair of stumps, representing the hinder limbs, remain.

No naturalist doubts that Seps and Bipes represent two stages of Degeneration, or atrophy of the limbs; that they have, in fact, been derived from the five-toed four-legged form, and have lost the locomotor organs once possessed by their ancestors. This very partial or local atrophy is not however that to which I refer when using the word Degeneration. Let us imagine this atrophy to extend to a variety of important organs, so that not only the legs, but the organs of sense, the nervous system, and even the mouth and digestive organs are obliterated—then we shall have pictured a thorough-going instance of Degeneration.

Degeneration may be defined as a gradual change of the structure in which the organism becomes adapted to less varied and less complex conditions of life; whilst Elaboration is a gradual change of structure in which the organism becomes adapted to more and more varied and complex conditions of existence. In Elaboration there is a new expression of form, corresponding to new perfection of work in the animal machine. In Degeneration there is suppression of form, corresponding to the cessation of work. Elaboration of some one organ may be a necessary accompaniment of Degeneration in all the others; in fact, this is very generally the case; and it is only when the total result of the Elaboration of some organs, and the Degeneration of others, is such as to leave the whole animal in a lower condition, that is, fitted to less complex action and reaction in regard to its surroundings, than was the ancestral form with which we are comparing it (either actually or in imagination) that we speak of that animal as an instance of Degeneration.

Any new set of conditions occurring to an animal which render its food and safety very easily attained, seem to lead as a rule to Degeneration; just as an active healthy man sometimes degenerates when he becomes suddenly possessed of a fortune; or as Rome degenerated when possessed of the riches of the ancient world. The habit of parasitism clearly acts upon animal organisation in this way. Let the parasitic life once be secured, and away go legs, jaws, eyes, and ears; the active, highly-gifted crab, insect, or annelid may become a mere sac, absorbing nourishment and laying eggs.

Reference was made above to the larval stage of a certain shrimp (Figs. 4, 5, 6). Let us now compare these with the young stages of a number of shrimp-like animals, viz., Sacculina, Lernæocera, Lepas, Cyclops, Limnetis, (all drawn in Fig. 4), some of which lead a parasitic life. All start equally with the recapitulative phase known as the Nauplius; but whilst the Nauplius of the free-living shrimp grows more and more elaborate, observe what happens to the parasites—they degenerate into comparatively simple bodies; and this is true of their internal structure as well as of their external appearance. The most utterly reduced of these parasites is the curious Sacculina (Fig. 7) which infests Hermit-crabs, and is a mere sac filled with eggs, and absorbing nourishment from the juices of its host by root-like processes.

Lernæocera again, which in the adult condition is found attached to the gills of fishes, has lost the well-developed legs of its Nauplius childhood and become an elongated worm-like creature (Fig. 8), fitted only to suck in nourishment and carry eggs.