Chapter V: EMBRYOLOGY. Page 131

I have not felt warranted to include in the text any summary of results secured by the important, but very difficult, investigations concerning the growth of animal life in the womb. This whole department of inquiry is in such an unfinished and uncertain state, that there is not warrant to found upon the evidence already obtained any general argument as to its bearing on a theory of evolution. The most competent observers admit that they are perplexed by facts ascertained, and confess that they can not as yet offer an explanation. To others all is as plain as possible; embryology supplies a convincing proof of the accuracy of an evolution theory; but these are scientific theorists who see by imagination, and are impatient of uncertainty. There are certain general considerations which must interpose difficulties in the way of constructing an argument from Embryology to evolution of species. (1) The action of environment before birth is altogether different from the action of environment after birth. (2) The theory of the evolution of species emphasizes this difference by insisting on the struggle for existence. (3) This difference being admitted, an argument from the one to the other can not hold. In the line of discovery the point of chief interest has been the fact that in some cases embryonic life shows a transition through lower forms analogous to lower orders of animal existence prior to reaching the mature stage when birth occurs. But in connection with the facts ascertained, two things are to be remarked. (1) Evidence of transition is most striking in the history of animal life developed external to the parental life, as in the transition from _larvæ_ to _pupæ_ among insects, and in the changes in the life of the tadpole. (2) If it be admitted that there is a common plan of structure for all organism, it is implied that there must be similarities in process of development. The question requiring answer, therefore, is whether in the gradual development from the germ, any further resemblance to lower orders appears than is to be anticipated on the admission of a common plan for organic structure. There are singular examples of transition. But there are no illustrations of uniform progress in the case of the higher orders such as would warrant the supposition that a history of evolution of the species can be read in the development of the foetus. The supposition has, however, found currency in not a little of our scientific teaching. The incompleteness of this evidence may appear from examples. Take the tadpole. Huxley states the facts thus,--"The tadpole is first a fish, then a tailed amphibian, provided with gills and lungs, before it became a frog." This is development outside parental life, and does not belong to evidence in Embryology. Confining attention to embryonic life, let us take Huxley's statement, biologists "trace back the dog's and the man's development, and they find that at a certain stage of their existence, the two creatures are not distinguishable the one from the other." What is the inference to be drawn? If the two are not distinguishable, our powers of distinguishing are insufficient, for no biologist suggests that the two are alike. The difficulty of distinguishing two germs, or two examples of foetus, is analogous to the difficulty which Darwin has pointed out of distinguishing the orders of dogs when they are six-days-old puppies, or the breed of three-days-old colt, or of nestling pigeons. At these stages, the animals may be so similar, that it is hardly possible to distinguish them, and yet in the full grown state they are quite different (Darwin's _Origin of Species_, sixth edition, p. 391). Mr. Darwin has presented the outstanding facts thus;--"The very general, though not universal, difference in structure between the embryo and the adult;--the various parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at an early period of growth alike;--the common, but not invariable resemblance between the embryos or larvæ of the most distinct species in the same class;--the embryo often retaining whilst within the egg or womb, structures which are of no service to it, either at that or at a later period of life."