EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 67 



of the recapitulation theory (p. 50) some biologists hailed the new idea as 

 a means of solving all enigmas of animal relationships and evolutionary 

 history (phylogeny). It was thought that if one studied the embryonic 

 development of a given species intensively enough one could trace step by 

 step the entire evolutionary history of that species. Hopes of doing this 

 dwindled when it was finally realized that adult ancestral stages are not 

 repeated by embryos, that the latter repeat embryonic conditions of their 

 ancestors only. Despite this restriction, however, recapitulation has much 

 to contribute to the study of evolution. 



Why does not an embryo repeat all the embryonic stages possessed by 

 its ancestors? The first answer is: lack of time. We have noted that most 

 of the recapitulation by the human embryo is confined to the first month, 

 practically all of it to the first two months, of life. How would it be pos- 

 sible in this brief span for the human embryo to retrace all the steps of 

 human evolution, involving many millions of years? 



Again, we must remember that the main "object" of an embryo is to be- 

 come an adult as expeditiously as possible. Retracing ancestral history is 

 purely secondary, of interest to inquiring biologists but not to the embryo 

 itself. Accordingly we may expect that whenever ancestral stages can be 

 condensed or entirely omitted by embryos such omissions and condensa- 

 tions will occur. The success of a species is dependent in part upon the 

 efficiency with which its embryos can become adults, without waste of time 

 or food supply. Inclusion of unnecessary embryonic stages would be waste- 

 ful in both respects and consequently a handicap to the species in life's 

 competition. Thus we may look upon the examples of recapitulation re- 

 maining to us as the irreducible minimum of ancestral stages which na- 

 ture has never found a way of eliminating or circumventing. It might 

 seem, for example, to increase embryonic efficiency if the muscles of the 

 human embryo could be formed in the first place as tiny miniatures of the 

 adult muscles, the somite stage described above being circumvented en- 

 tirely. But apparently this has not been possible or feasible. We may be 

 reasonably certain that if it had been possible to omit somite formation we 

 should not find the latter as a constant feature of embryonic development 

 in amphibians, reptiles, birds, and mammals. But many other features of 

 embryos of ancestral forms must have been lost to us because it was pos- 

 sible for the embryos of their descendants to omit them. 



Why do embryos of higher animals ever find it necessary to retain struc- 

 tures characteristic of the embryos of their ancestors? The more we learn 

 of forces operative in embryonic development the more we realize that 

 the whole process is like a chain reaction, starting before fertilization of 



