CHAPTER XVII.

THE PLASTICITY AND THE PERMANENCY OF CHARACTERS IN THE HISTORY OF ORGANISMS.

Races in Paleontology.-During the life-period of a genus constant changes are found to take place among the representatives of the genus as we follow them upward from stage to stage of their geological succession. The forms appearing at the first epoch, in the life-period of a genus, are generally found to be of different species from those occurring later; and in many genera there are enough specimens collected, and sufficient knowledge regarding them accumulated, to enable the paleontologist to recognize a series of forms regularly succeeding one the other, presenting slight modification from one stage to the next, but those of each stage showing closer resemblance to those immediately preceding them than to any other species of the same genus. The series of forms thus resembling each other may be called races, because of the very evident genetic relationship existing between the later and the earlier representatives of the series.

Phylogeny of the Race. When we examine the details of form in such a series of succeeding forms or races of a genus, comparatively, it is often apparent that the changes undergone in respect to each character are progressive or of an accumulative nature, and thus they resemble the changes which the individual undergoes in ordinary growth. The technical name proposed by Haeckel for this morphological history of the race is Phylogeny, contrasting it with Ontogeny or the history of growth or development of the individual, from its relatively homogeneous condition in the ovum to the more or less differentiated adult organism.

Mutability and Phylogeny.-The Cuvierian school of naturalists believed in the immutability of species, and for them

the principle of racial evolution or phylogeny was barred out. But Geoffroy St. Hilaire and Lamarck with their idea of mutability of species laid the way for a consistent theory of phylogenetic evolution, although in their time the knowledge of paleontology was not far enough advanced to furnish actual phylogenetic series of organisms. It was, however, not till Darwin had constructed a working hypothesis for the steps and manner by which new types of organisms can arise, that evolution became an accepted mode of explanation of the course of biological history.

The great advance which the present generation has witnessed in the interpretation of the science of organisms is the change in belief, which all naturalists have more or less thoroughly undergone, from the doctrine of immutability to that of mutability of species. Some theory of evolution and phylogenetic origin of species is the necessary outcome of this new doctrine. Darwin more than any other single man was the means of producing the change of conviction in regard to this point.

The Phylogenetic Theory of Evolution.-The phylogenetic theory of evolution is logically an expansion and application of the principle of organic growth, already recognized in the development of individual characters, to the evolution of specific and more fundamental differences. It is a recognition of an organic correlation between separate individuals. As growth takes place in the individual by the segmentation and separation of cells, with specialization of functions, first for different cells and finally for the complex structures called organs, the whole showing its organic unity by the mutual coöperation of all of the parts in the life of the whole, so the phylogenetic theory recognizes in the species, or the race of species, an organic unity of a higher sphere, in which the individuals play the part of mutually adjusted and coöperating parts in this greater organic whole.

The theory goes one step further, and includes the proposition, that as the principle omne vivum ex ovc is true in the life-history of individuals, so each species postulates a preexisting species. This is the philosophy of the theory, but it must be observed that the concrete facts illustrating these

laws are always found together in the same organism. Each individual organism is the source and record of those facts which we separately interpret as evidence of cell-growth, individual growth, the differentiation of organs, and the phylogenetic evolution.

Thus there are series of organic forms succeeding each other in some regular order, known or unknown, which are bound together by organic, and in this case called particularly genetic relationship. The changes in form observed upon comparing the individuals at different points in the line of succession are accounted for by some law of evolution, and the origin of the different members of the series is said to be by generational descent, the later arising from the earlier. On account of the mutability of form in the process, species presenting different form, different function, and incapable of organic fertility are supposed to have arisen originally from a common parentage.

Mutability the Fundamental Law of Organisms; the Acquirement of Permanency Secondary. This analysis brings us face to face with one of the chief inconsistencies in the prevalent conception of the nature of organisms. While the doctrine of mutability of species has generally taken the place of immutability, the proposition that like produces like in organic generation is still generally, and I suppose almost universally, accepted. It therefore becomes necessary to suppose that variation is exceptional, and that some reason for the accumulation of variation is necessary to account for the great divergencies seen in different species. Darwin's theory of natural selection is chiefly concerned in accounting for the accumulation, increase, and perpetuation of divergencies arising by natural variation.

If we extend the principle of mutability, and instead of regarding it as an accidental circumstance in the life-history of organisms, recognize it as the distinctive and fundamental characteristic of living beings, we escape this inconsistency.

In the physical and chemical world like causes do produce like effects; but in the organic world like produces like "with an increment," as Professor J. D. Dana put it. Mutability and variation are evidences of this increment.

The

increment is the great fact; the checking and limiting of it is secondary. The search has been for some cause of the variation; it is more probable that mutability is the normal law of organic action, and that permanency is the acquired law.

It is more probable that the use and tested adaptability of a variable part is the cause of checking the variability and of the transmission of the character with less or no variation, than that the variation is increased by this process. Adopting mutability as a fundamental law of all organic activity, and the construction of a theory of evolution becomes a simple matter. We have in that case to account for the acquirement of permanency of characters. This is found in the principle of ordinary generation, the instituting of habit, and the more and longer the species breed together the closer and more fixed will the characters become.

Early Plasticity Succeeded by Permanency expressed in Geological History.-Examination of the history of geological species suggests the truth of this hypothesis, for it is observed that many species, which by their abundance and good preservation in fossil state give us sufficient evidence in the case, exhibit greater plasticity in their characters at the early stage than in later stages of their history. A minute tracing of lines of succession of species shows greater plasticity at the beginning of the series than later, and this is expressed in the systematic description and tabulation of the facts by an increase in the number of the species.

In order to illustrate this law a special consideration will now be given to the facts regarding the laws of specific history as observed by the paleontologist.

Pritchard's Definition in which the Constancy of Transmission of Same Peculiarity is made the Criterion of Species. Thus far we have been considering generic characters-that is, those characters which are constant for one or more species. The next question to consider is. What are the laws exhibited in the history of specific characters? There are various definitions of species which are more or less theoretical; but whatever our theory about the definition, the fact remains that all naturalists do recognize within slight limits of difference the reality of groups of organisms called by the name species.

In a previous page are given some of the definitions of species formulated by early naturalists. Alfred R. Wallace, who published as early as 1855 an article on the law which has regulated the introduction of new species (Darwin's “Origin of Species" was published in 1859), set forth some of the chief principles of the modern evolutionary conception of the history of organisms. Wallace made a careful study of species, and, perhaps as well if not better than any one else, understands the relationship between species and geographical distribution. In an article of his "On the Malayan Papilionidæ, or Swallow-tailed Butterflies, as Illustrative of the Theory of Natural Selection," published in 1864, is found the following definition of the word species:*

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"In estimating these numbers [of the species of Papilionidæ] I have had the usual difficulty to encounter, of determining what to consider species and what varieties. The Malayan region, consisting of a large number of islands of generally great antiquity, possesses, compared to its actual area, a great number of distinct forms, often indeed distinguished by very slight characters, but in most cases so constant in large series of specimens, and so easily separable from each other, that I know not on what principle we can refuse to give them the name and rank of species. One of the best and most orthodox definitions is that of Pritchard, the great ethnologist, who says that separate origin and distinctness of race, evinced by a constant transmission of some characteristic peculiarity of organization, constitutes a species. Now leaving out the question of · origin,' which we cannot determine, and taking only the proof of separate origin, the constant transmission of some characteristic peculiarity of organization,' we have a definition which will compel us to neglect altogether the amount of difference between any two forms, and to consider only whether the differences that present themselves are permanent. The rule, therefore, I have endeav-. ored to adopt is, that when the difference between two forms inhabiting separate areas seems quite constant, when it can be defined in words, and when it is not confined to a single peculiarity only, I have considered such forms to be species. When, however, the individuals of each locality vary among themselves, so as to cause the distinctions between the two forms to become inconsiderable and indefinite, or where the differences, though constant, are confined to one particular only, such as size, tint, or a single point of difference in marking or in outline, I class one of the forms as a variety of the other. I find as a general rule that the constancy of species is in inverse ratio to their range. . . . When a species exists over a wide area, it must have had, and probably still possesses, great powers of dispersion. . . . When, however, a species has a limited range, it indicates less active powers of dispersion, and the process of modification under changed

* 'Contributions to the Theory of Natural Selection. A Series of Essays." P. 141. Macmillan & Co., 1870.