Ix YOLK: KECAPITULATION 491 



ontogeny the developing individual tends to progress constantly 

 towards the goal of adult structure. Not in this case however 

 necessarily by the straightest and shortest path. The structure 

 of the adult is the expression of the action of Heredity. The 

 earlier stages are not exempt from the same influence. Each step 

 in the development of the ancestor tends to be repeated in the 

 development of the descendant. The descendant then during its 

 ontogeny tends to pursue the same, it may be devious, path as the 

 ancestor. If in the course of generations the adult structure becomes 

 shifted onwards in a process of evolution, this merely means the 

 adding on of a new portion at the latter end of the ontogenetic path. 

 The earlier portions of this path, built up of similar increments 

 representing previous steps in evolutionary progress, are repeated 

 as before, and so the complete process of individual development 

 forms a record or recapitulation of phylogenetic history. 



It cannot be too constantly borne in mind that the factor just 

 indicated is the supreme factor in ontogenetic development. Other 

 factors may be superficially conspicuous, may have far-reaching 

 influence upon details, but this factor — the tendency to repeat 

 ancestral steps in development up to and including the final char- 

 acters of the adult — is and must always be paramount. 



Modern advances in. knowledge of the facts of embryology, 

 together with the assumption of a properly critical frame of mind, 

 have shown, however, that the picture of past evolution afforded by 

 the phenomena of individual development is at the best but a blurred 

 and imperfect one, and that this must necessarily be so is readily 

 realized when we remember that a large proportion of the characters 

 of any organism are adaptive to its special mode of life. The 

 circumstances under which a developing organism exists are, as a 

 rule, widely different from those under which its ancestors proceeded 

 along the evolutionary path, and in correlation with this its adaptive 

 features are equally distinct. As we study the development of any 

 species of animal we do not then see before us a complete and perfect 

 picture of its evolutionary history, but merely gain fleeting, and it 

 may be misleading, glimpses through the obscuring clouds of 

 adaptive features. 



A further disturbing factor is indicated by the consideration 

 that in past evolutionary history each stage in evolution was repre- 

 sented by a complete functional organism, all the parts of which 

 were necessarily at correlated stages of development so as to form a 

 functional whole. Many modern animals however develop under 

 conditions in which the different systems of organs are no longer 

 forced to keep accurate step with one another, and the result is that 

 some lag behind while others, particularly organs of great histological 

 complexity in the adult — such as the brain or the eye — are accelerated 

 in their early development, so as to give time for the complicated 

 histogenetic processes that have to be completed before the organ 

 can become functional. It will be realized that this latter type of 



