EMBRYOLOGICAL CRITERION OF HOMOLOGY. 121 



They indicate, farther, that the ontogenetic stages are plastic, 

 capable of modification, in a far higher degree than has hitherto 

 been supposed ; and they point towards the conclusion that 

 the events of ontogeny are essentially adaptive, and that the 

 persistence of ancestral reminiscences in development or of simi- 

 larities in the development of homologous parts is in some way 

 connected with the persistence of ancestral conditions of develop- 

 ment. We are still too ignorant of the nature of these condi- 

 tions to make much use of this conclusion, but the way of 

 further investigation is pointed out by two recently enunciated 

 principles. The first of these has recently been stated by 

 Adam Sedgwick, who, arguing along very different lines from 

 those I have followed, concludes that " the tendency in embry- 

 onic development is to directness and abbreviation; that ances- 

 tral stages of structure are only retained in larval stages in so 

 far as they are useful ; and that their appearance in the embry- 

 onic (foetal) stages is owing to * the absorption of a larval or 

 immature free stage into embryonic life,' where they become 

 • functionless,' and therefore largely removed from the direct 

 action of natural selection." This is undoubtedly a true ex- 

 planation as far as the larval stages are concerned, and in a 

 measure, no doubt, applies to the embryonic stages. It leaves 

 out of account, however, a second principle which was enunci- 

 ated by Kleinenberg (1886), namely, that although embryonic 

 ancestral stages may be functionless so far as the external envi- 

 ronment of development is concerned, they are still functional 

 in the sense of forming a more or less necessary part of the 

 mechanism of development, — i.e., as a preparation for organs 

 that succeed them in the ontogeny, as, for example, cartilage 

 precedes bone, or a tubular heart forms the foundation for a 

 chambered one. " From this point of view many rudimentary 

 organs appear in a different light. Their obstinate reappear- 

 ance throughout long phylogenetic series would be hard to 

 understand were they really no more than reminiscences of by- 

 gone and forgotten stages. Their significance in the processes 

 of individual development may in truth be far greater than is 

 generally recognized. When in the course of the phylogeny 

 they have played their part as intermediary organs ( Vermittel- 



