Fig. 26.—Antlers of Stag, showing successive addition of branches in successive years. Drawn from nature (Brit. Mus.).
In the only illustration hitherto adduced—viz. that of deers’ horns—the series of changes from a one-pronged horn to a fully developed arborescent antler, is a series which takes place during the adult life of the animal; for it is only when the breeding age has been attained that horns are required to appear. But seeing that every animal passes through most of the phases of its development, not only before the breeding age has been attained, but even before the time of its own birth, clearly the largest field for the study of individual development is furnished by embryology. For instance, there is a salamander which differs from most other salamanders in being exclusively terrestrial in its habits. Now, the young of this salamander before their birth are found to be furnished with gills, which, however, they are never destined to use. Yet these gills are so perfectly formed, that if the young salamanders be removed from the body of their mother shortly before birth, and be then immediately placed in water, the little animals show themselves quite capable of aquatic respiration, and will merrily swim about in a medium which would quickly drown their own parent. Here, then, we have both morphological and physiological evidence pointing to the possession of gills by the ancestors of the land salamander.
It would be easy to devote the whole of the present chapter to an enumeration of special instances of the kinds thus chosen for purposes of illustration; but as it is desirable to take a deeper, and therefore a more general view of the whole subject, I will begin at the foundation, and gradually work up from the earliest stages of development to the latest. Before starting, however, I ask the reader to bear in mind one consideration, which must reasonably prevent our anticipating that in every case the life-history of an individual organism should present a full recapitulation of the life-history of its ancestral line of species. Supposing the theory of evolution to be true, it must follow that in many cases it would have been more or less disadvantageous to a developing type that it should have been obliged to reproduce in its individual representatives all the phases of development previously undergone by its ancestry—even within the limits of the same family. We can easily understand, for example, that the waste of material required for building up the useless gills of the embryonic salamanders is a waste which, sooner or later, is likely to be done away with; so that the fact of its occurring at all is in itself enough to show that the change from aquatic to terrestrial habits on the part of this species must have been one of comparatively recent occurrence. Now, in as far as it is detrimental to a developing type that it should pass through any particular ancestral phases of development, we may be sure that natural selection—or whatever other adjustive causes we may suppose to have been at work in the adaptation of organisms to their surroundings—will constantly seek to get rid of this necessity, with the result, when successful, of dropping out the detrimental phases. Thus the foreshortening of developmental history which takes place in the individual lifetime may be expected often to take place, not only in the way of condensation, but also in the way of excision. Many pages of ancestral history may be recapitulated in the paragraphs of embryonic development, while others may not be so much as mentioned. And that this is the true explanation of what embryologists term “direct” development—or of a more or less sudden leap from one phase to another, without any appearance of intermediate phases—is proved by the fact that in some cases both direct and indirect development occur within the same group of organisms, some genera or families having dropped out the intermediate phases which other genera or families retain.
The argument from embryology must be taken to begin with the first beginning of individual life in the ovum. And, in order to understand the bearings of the argument in this its first stage, we must consider the phenomena of reproduction in the simplest form which these phenomena are known to present.
The whole of the animal kingdom is divided into two great groups, which are called the Protozoa and the Metazoa. Similarly, the whole of the vegetable kingdom is divided into the Protophyta and the Metaphyta. The characteristic feature of all the Protozoa and Protophyta is that the organism consists of a single physiological cell, while the characteristic of all the Metazoa and Metaphyta is that the organism consists of a plurality of physiological cells, variously modified to subserve different functions in the economy of the animal or plant, as the case may be. For the sake of brevity, I shall hereafter deal only with the case of animals (Protozoa and Metazoa); but it may throughout be understood that everything which is said applies also to the case of plants (Protophyta and Metaphyta).
A Protozoön (like a Protophyton) is a solitary cell, or a “unicellular organism,” while a Metazoön (like a Metaphyton) is a society of cells, or a “multicellular organism.” Now, it is only in the multicellular organisms that there is any observable distinction of sex. In all the unicellular organisms the phenomena of reproduction appear to be more or less identical with those of growth. Nevertheless, as these phenomena are here in some cases suggestively peculiar, I will consider them more in detail.
A Protozoön is a single corpuscle of protoplasm which in different species of Protozoa varies in size from more than one inch to less than 1/1000 of an inch in diameter. In some species there is an enveloping cortical substance; in other species no such substance can be detected. Again, in most species there is a nucleus, while in other species no such differentiation of structure has hitherto been observed. Nevertheless, from the fact that the nucleus occurs in the majority of Protozoa, coupled with the fact that the demonstration of this body is often a matter of extreme difficulty, not only in some of the Protozoa where it has been but recently detected, but also in the case of certain physiological cells elsewhere,—from these facts it is not unreasonable to suppose that all the Protozoa possess a nucleus, whether or not it admits of being rendered visible by histological methods thus far at our disposal. If this is the case, we should be justified in saying, as I have said, that a Protozoön is an isolated physiological cell, and, like cells in general, multiplies by means of what Spencer and Häckel have aptly called a process of discontinuous growth. That is to say, when a cell reaches maturity, further growth takes place in the direction of a severance of its substance—the separated portion thus starting anew as a distinct physiological unit. But, notwithstanding the complex changes which have been more recently observed to take place in the nucleus of some Protozoa prior to their division, the process of multiplication by division may still be regarded as a process of growth, which differs from the previous growth of the individual cell in being attended by a severance of continuity. If we take a suspended drop of gum, and gradually add to its size by allowing more and more gum to flow into it, a point will eventually be reached at which the force of gravity will overcome that of cohesion, and a portion of the drop will fall away from the remainder. Here we have a rough physical simile, although of course no true analogy. In virtue of a continuous assimilation of nutriment, the protoplasm of a cell increases in mass, until it reaches the size at which the forces of disruption overcome those of cohesion—or, in other words, the point at which increase of size is no longer compatible with continuity of substance. Nevertheless, it must not be supposed that the process is thus merely a physical one. The phenomena which occur even in the simplest—or so-called “direct"—cell-division, are of themselves enough to prove that the process is vital, or physiological; and this in a high degree of specialization. But so, likewise, are all processes of growth in organic structures; and therefore the simile of the drop of gum is not to be regarded as a true analogy: it serves only to indicate the fact that when cell-growth proceeds beyond a certain point cell-division ensues. The size to which cells may grow before they thus divide is very variable in different kinds of cells; for while some may normally attain a length of ten or twelve inches, others divide before they measure 1/1000 of an inch. This, however, is a matter of detail, and does not affect the general physiological principles on which we are at present engaged.