12 The Recapitulation Theory and Human Infancy 



of Darwin. The contribution of Fritz M tiller to the classifica- 

 tion of the Crustacea, largely by the aid of embryology, had 

 been of particular interest to him, and Haeckel's work with 

 phylogenies was alluded to with approval. 15 



Darwin's treatment of the facts under discussion is found in 

 Chapter XIV of the sixth edition where he deals with the mutual 

 affinities of organic beings. Referring to the efforts of the 

 "special creationists" to arrange a " natural system" in ac- 

 cordance with the divine plan, he urges that such a plan is mean- 

 ingless unless it relates to time and place and has a specific con- 

 tent, and that all the rules, aids, and difficulties in the then 

 current efforts at classification are well explained by a view of 

 a natural system founded on descent with modification. In 

 support of this contention he proceeds to submit to scrutiny the 

 several kinds of recognized affinities among animals, those of 

 embryology among them. After a citation of facts, including 

 some of von Baer and a direct reference to his law of embryonic 

 resemblance, Darwin asks, 



"How then can we explain these several facts in embryology, 

 namely, 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 larvae 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; on the other hand larvae, which have 

 to provide for their own wants, being perfectly adapted to the 

 surrounding conditions; and lastly the fact of certain larvae 

 standing higher in the scale of organization than the mature 

 animal into which they are developed?" 



The explanation is contained in the two principles, viz., 

 " . . . that slight variations generally appear at a not very 

 early period of life and are inherited at a corresponding not 

 early period." Darwin's illustration helps to make his meaning 

 plain, 



"Now let us apply these two principles to species in a state 

 of nature. Let us take a group of birds, descended from some 

 ancient form and modified through natural selection for differ- 



Origin of Species, American ed., p. 231. 



