Embryos and Fossils 139 



A study of the life history, however, shows very clearly that 

 it too is a crustacean in the early stages and degenerates into 

 an almost structureless sac, after adopting the parasitic 

 habit (see page 108 and Fig. 33). 



Are we to classify the barnacle and the Sacculina on 

 the basis of their adult form, or is it reasonable to take into 

 consideration the entire life history? If we do the latter, 

 we assign them a place among the arthropods which they 

 do not at all resemble as adults. If, however, we mav accept 

 the embryonic stages as indicative of true relationship in these 

 animals, we must consistently do the same in other groups. 



Embryos and Fossils 



We cannot escape the fact of similarity between the 

 embryonic stages of animals that in the adult represent dis- 

 tinct plans of organization. We may not go so far as 

 to say with Haeckel that ontogeny or individual becoming 

 repeats phylogeny or the evolutionary history of the race, 

 except in a most general sense. It is sufficient to see that 

 embryos of recent and more highly specialized animals are 

 survivals corresponding to the embryonic structure of lower 

 forms. We need not say that the human embryo passes 

 through a fish stage. It is enough to say that both the human 

 embryo and the fish embryo, as well as the reptilian and bird 

 embryos, pass through similar stages. 



There are, however, striking similarities between the 

 embryos of higher animals and the adults of earlier levels of 

 organic evolution. Louis Agassiz, who like his master Cuvier 

 was opposed to the idea that species change, or that present- 

 day forms arose by diverging from ancestors in structure, 

 nevertheless pointed out that the stages in the development 

 of all living animals correspond to the order of succession 

 of their extinct representatives in geologic times. This was 

 in 1858, the year before Darwin published his Origin of 

 Species. His own son subsequently confirmed this gen- 

 eralization by comparing in detail the series of fossil sea 



