CHAPTER IV 



EMBRYONIC AND LARVAL HISTORIES 



THE doctrine that animals in the course of their 

 development pass through or recapitulate the stages 

 of their ancestral history, in other words that animals 

 in their development climb up their own genealogical 

 trees, has given rise to as much controversy as any 

 biological theory. Von Baer, who is generally held 

 responsible for this doctrine, did not enunciate it in 

 this form ; his statement was that in any two or more 

 related animals the further back we go in their 

 developmental history from the egg the more do 

 they resemble one another. In this form it must be 

 admitted that Von Baer's law holds good with very few 

 exceptions, but the extension of this generalization to 

 mean that the developmental stages represent actual 

 adult animals, the ancestors of the particular species 

 in question, is open to very grave objections and is 

 indeed only partially true in a few cases. The fact 

 that related animals on the whole differ less in their 

 developmental stages from one another than in their 

 adult form indicates that on the whole the process 

 of structural change and evolution has aifected the 

 developmental and larval histories of animals less 



