Part 5

By virtue of the automatic variation of the idioplasm the ontogenies of a phylogenetic line attain to a continually more complex organization and greater differentiation of function. In this differentiation, however, only the qualitative differences are of importance; quantitative and numerical gradations may be disregarded. The more complex admits of more combinations than the simpler; hence if a phylogeny reaches a higher stage by automatic evolution it may branch into several lines, of which each appears as the continuation of the parent stock.

Since adaptive variations depend only on the transmutations of environment, an organism may rise to a higher organization and division of labor by continually adapting itself to the changed environment. But the organism may also change its adaptation while it remains at the same stage of organization. And since the adaptive variation is quickly perfected as compared with automatic evolution, although extremely slowly as compared with the duration of the ontogeny, an organization may change its adaptation several times while it remains at the same grade of organization and division of labor. Since there are also numerous different kinds of adaptation, a phyletic line may divide at each point into several adaptive forms, which appear in the taxonomic system as species, genera, often even as whole families, while in other cases various degrees of organization have appeared in one family.

20. LAWS OF EVOLUTION OF THE PLANT KINGDOM.

In the sub-organic kingdom, which precedes the plant and animal kingdoms, (see page 5), there are gradually formed from the spontaneously generated plasma independent cells with their characteristic properties, i.e., growth by intussusception of micellae, formation of a plasmic cuticle, and a non-plasmic membrane about the same, division of the cells, separation of the cells thus formed, and free cell formation within the cell contents. These properties are inherited from the sub-organic kingdom by the plants and animals which follow in the next stage of phylogeny. The evolution of the plant kingdom proceeds through the following regular processes, which continue to operate through the entire phylogenetic series.

Law of Phylogenetic Combination.--The simplest of all plants are cells of round form, which grow and reproduce themselves by division, budding or free cell formation. From the fact that the younger generation of cells, instead of separating from each other and growing to independent plant individuals, remain united with each other, multicellular plants arise from unicellular. The same transformation of the reproductive cells into non-separable tissue cells is repeated several times in multicellular plants and serves to enlarge the individual. There is manifested in this phylogenetic process the tendency of the plant to combine in the higher stages into one complex whole those parts which in the lower stages tend to be independent. A similar unifying tendency is revealed also in those plant members which have arisen by differentiation and represent a system only by their being connected at certain points. These combine in the higher stages and form ultimately continuous tissues.

Law of Phylogenetic Complication or Ampliation, Differentiation and Reduction.--The cells, and, in general, the parts of plants which lie near each other in space or follow upon each other in time, are always alike in the lower stages. By differentiation they become unlike, so that the sum of the functions which at first fall to the lot of all parts without distinction now is shared among the individual parts. By this means each part can perform its own special function so much the better. Differentiation is repeated in the course of the phylogeny, since at first all parts of an ontogeny diverge into two or more parts, then the parts of these parts divide again, etc. Along with this process of division another process is always active, which, as it were, prepares the way for the former, namely, ampliation, by virtue of which the growth of the whole ontogeny or of single stages of it undergoes a quantitative increase, so that an organ acquires a greater number of cells, and an individual a greater number of organs. After this increase in number of parts in a stage of ontogeny, differentiation follows as far as the nature of the functions permits, by the parts most separated passing into each other by intermediate gradations. By the further phylogenetic process of reduction the intermediate forms are suppressed. At last only the extreme products of differentiation lie near each other in space or follow upon each other in time; and these products are as limited in quantity and number as possible.

Along with the above named phylogenetic processes, which take place by the automatic increase of the idioplasm, external influences are always active. These lend to the organism at times a local stamp corresponding to its environment, and follow the law of adaptation.

21. ALTERNATION OF GENERATIONS IN RELATION TO PHYLOGENY.

Since the simplest plants are cells and the more complex ones are formed from cells, a whole phylogenetic line may be regarded as a series of cell generations following one after another. In the lowest forms all cell generations are like each other; in all others they show differences which become continually greater and more numerous. Thus alternation of generations in cells exists, because the successive generations become more and more complicated at each succeeding period. Among these periods the ontogenetic period or ontogeny embraces all generations from one cell to the return of the exactly similar kind of cell. In the lowest forms of cell differentiation the cells of successive generations are all independent; the ontogenetic period consists of a cycle of generations of unicellular plants. Later the cell generations of an ontogeny are united by parts into plant individuals; the ontogenetic period consists of a cycle of multicellular and unicellular, or only of multicellular plant generations. If all the cell generations of an ontogenetic period have been united into a single individual, the successive plant generations are alike and alternation of generations has ceased.

The unlikeness of the generations arises either from inner causes of temporary differentiation alone, or by temporary differentiation which receives a definite imprint by the change of seasons. But in the latter case the characteristic of adaptation is again lost in the course of the phylogeny and alternation of generations follows then without regard to the season. If the given adaptation is united in the lower plants with alternation of generations during the ontogenetic periods, one of the unlike plant generations is repeated an indefinite number of times (repetitional generation), while the other unlike plant generation appears only once and then at the beginning of the resting stage and remains latent in the form of a resting spore till the beginning of the next period of generation. With this peculiar transition generation, which has arisen in the lower stages asexually, and in the following higher stages by the union of a male and a female cell, and which hence is hermaphrodite, there are generally associated later two other single generations--viz., a generation preceding and one following the hermaphrodite, the former as a sex-producing generation, the other as a sex-produced generation.

The phylogenetic significance of the alternation of generations consists in its representing a transition stage from the unicellular to the simpler multicellular and from the latter to the more complex multicellular plants. The plant generations of any phylogenetic stage increase by ampliation, become unlike by differentiation in time (alternation of generations), and unite in a plant individual, whose unlike ontogenetic stages correspond to the unlike plant generations of the earlier ancestral series.

22. MORPHOLOGY AS THE SCIENCE OF PHYLOGENY.

All organic phenomena belong, according to their causes, to two different classes: (1) Those belonging to one group are the results of external influences in each ontogeny and are not inherited; they represent nutrition varieties, are experimentally demonstrable, and constitute the subject matter of experimental physiology. (2) The others are inherited and again transmitted; they belong to the physiology of the idioplasm. This subject is mainly occupied with the origin of the determinants, hence with the formation of varieties and species. It is not the subject of experiment, and constitutes the phylogeny or the physiology of the formation of determinants. A sub-division of this subject is occupied with the development of the determinants already present, hence with the formation of races. It is elucidated especially by experiments in crossing and may be designated as the physiology of the development of the determinants.

The morphological phenomena which find their application in taxonomy, belong exclusively to phylogeny. Their ontogenetic history does not explain their true significance; this can be known only in a phylogenetic way by comparison of one phenomenon with those phenomena from which it has arisen in the course of evolution.

23. PLANT CLASSIFICATION FROM THE STANDPOINT OF PHYLOGENY.

Spontaneous generation has taken place at all times and in all places, in as far as the necessary conditions were concurrently present. (See page 47). After spontaneous generation the automatic phylogenetic evolution begins and advances constantly. Consequently the phylogenetic line rises from time to time to higher stages of organization and division of labor, but dies of old age if the automatic perfecting process ceases. The phylogenetic lines of organisms now living have therefore an unequal age; those of the most highly developed plants and animals had their origin in the earliest periods of organic life, those of the lowest organisms in the most recent periods. Hence no general genetic relation exists among lines now living; only those that are nearly related and have reached approximately equal stages of organization may be regarded as branches of the same phylogenetic stock. A phylogenetic plant system does not exist in fact, but only in figure.

If genetic relation between two races is assumed, either as a reality or as a symbol, the degree of relationship is determined in a theoretically exact manner by the number and length of the phylogenetic steps which are found either between them both or between them and the common starting point, according as races belong to the same or collateral lines. The fact that two organisms belong to the same line of descent is recognized from the ontogeny of the higher including the ontogeny of the lower.

Since only a proportionately small number of known forms can appear as types of the supposed stages of evolution, only a few phylogenetic lines, and these only in a general way, may be established, on account of the great incompleteness of the present plant world. Such a line proceeds from the green filamentous algae through the liverworts to the vascular plants. Among the phanerogams, apparently so numerously represented, only phylogenetic series of individual organs can be ascertained, but no phylogenetic series of families. A phylogenetic system of phanerogams is not to be hazarded in the roughest outline. Even the relative rank of the two chief divisions of the angiosperms, the monocotyledons and dicotyledons, is a matter of question, as also which family in each of these divisions is to be considered the most perfect.

APPENDIX.

TRANSLATORS' NOTES.

The Mechanico-physiological Theory of Evolution, (Mechanisch-Physiologische Theorie der Abstammungslehre), by Carl von Naegeli, was published in Munich and Leipsic in 1884 in a large octavo volume of 822 pages, including two large appendices. The Abstammungslehre proper, including the summary, occupies 552 pages, and constitutes, in its way, one of the most important contributions to theoretical biology. It is difficult to understand how a work of so much consequence should have received such comparatively small notice in this country, especially as Naegeli's theories seemed calculated by nature to appeal much more strongly to American students than do, for instance, those of Weismann, who has been studied ten times as much as Naegeli. This is doubtless due, in part, to the fact that we have had no English translation of Naegeli's work, a circumstance much to be regretted.

The foregoing translation of the summary from Abstammungslehre goes but a small way toward making Naegeli's theories accessible to English-reading students, but it will, at least, be better than nothing. The work covers a great range of subjects, all, however, having a certain relationship to each other. In the main part of the book the discussion is presented in the following order: (1) Idioplasm as bearer of the inheritable determinants; (2) Spontaneous generation; (3) Causes of variation; (4) Determinants and visible characters, in which the origin and function of the determinants is presented; (5) Variety, race, "nutrition variety," heredity and variation; (6) Criticism of the Darwinian theory of natural selection, in which the author urges seven objections to that theory; (7) Laws of evolution of the plant kingdom; (8) Alternation of generations from the standpoint of phylogeny; (9) Morphology and classification as phylogenetic sciences; (10) A comprehensive summary of the whole work, a translation of which is given in the foregoing pages.

In the first part of the work Naegeli sets forth his micellar theory of the structure of organized bodies. This is one of his most important contributions to science. Until recent years it has been the only theory given in botanical text-books. At the present time its only competitor is Strasburger's lamellar theory, and even this has not superseded Naegeli's work to any great degree.

The reader who may not be familiar with the micellar theory will find the general idea from the following brief sketch adapted from Vines's Plant Physiology:

"Naegeli's micellar theory was developed from his study of organized bodies, especially of cell walls and starch grains. From the behavior of organized substance toward water absorbed by it, he concluded that water does not penetrate into the micellae, but only among them, thus merely separating them more from each other. He reasoned that if water should penetrate into the micella, its structure would be disintegrated. Hence he argued that organized bodies consist of solid micellae, which, with their respective films of water, are held together by: (1) The attraction of the micellae for each other, which varies inversely as the square of the distance. (2) The attraction of the micellae for water, which varies inversely as some higher power of the distance. (3) The force which holds together the ultimate chemical molecules of which each micella consists.

"Since the swelling up of organized bodies does not take place equally in all three dimensions of space, and on account of their double refraction, Naegeli inferred that in form the micellae are crystals, probably parallelopipedal, with rectangular or rhomboidal bases."

The law that "bodies attract each other with a force which varies inversely as the square of the distance," has been proven only in its application to the heavenly bodies. Naegeli has applied this law to molecules, unsupported, however, by any evidence other than that of analogy. On the other hand, there is evidence that molecules do not invariably act according to this law.

Spontaneous generation (p. 4) was an important item in Naegeli's doctrine, and might almost be said to be fundamental to it, although it is not really necessary to the internal perfecting principle, which may be regarded as the chief feature of the Mechanico-Physiological Theory. Up to 1865 Naegeli believed in the spontaneous origin of many fungi, and thought that it could be demonstrated. He was obliged to abandon the experimental evidence, but to the close of his life held the views of abiogenesis presented in the accompanying translation.