ONTOGENY 63 



stage in complexity is an aggregate of cells usually in 

 the form of hollow balls, and then comes a series of 

 animals closely resembling the two-layered stage in the 

 embryo. These are in exact correspondence with the 

 first three stages in the development of any individual 

 organism. From these facts von Baer and Meckel and 

 several others were led to examine the whole develop- 

 ment of an individual, and they came to the conclusion 

 that there was a tendency for the individual to recapi- 

 tulate certain stages in the history of the race. Haeckel, 

 a strong supporter of this view, called it the Recapi- 

 tulation Doctrine or the Biogenetic Law. 



Considerable importance has been placed on this doc- 

 trine, and in the popular mind it has been twisted to 

 mean very many impossible things. All workers in 

 Biology are agreed that there is no completeness of 

 recapitulation, that there is only a tendency to reproduce 

 in the individual life-history certain stages in the life- 

 history of the race, and so the law may be stated as 

 follows, that ontogeny tends to recapitulate pJiylogeny. 



An example or two of this tendency must suffice. 

 The heart in the higher animals, in development, is first 

 noted as a simple straight tube similar to that of many 

 invertebrates, then it becomes separated into an anterior 

 portion or ventricle, and a posterior or auricular portion, 

 resembling at this stage the heart of some of the fishes. 

 Later the posterior portion becomes separated into two, 

 and this corresponds in the main to the heart of some 

 amphibians. Much later it develops by bending and 

 fusion into a four-chambered organ in which the portion 

 that was originally posterior now becomes anterior, and 



