EVOLUTION 407 



Homology (similarity of structural development) is to be regarded 

 as a sign of relationship, as it is assumed that in such cases little or no 

 structural change has taken place in times past. Contrariwise, organ- 

 isms which are quite dissimilar in structure are assumed to have diverged 

 many ages ago. 



4. Comparative Embryology. 



(a) Likenesses between embryos of different animals are assumed 

 to demonstrate a close fundamental relationship and a common ancestor. 

 An example often quoted is that of Sacculina (Fig. 212), a parasite on 

 the abdomen of the crayfish. This parasite is merely a rounded, pulpy 

 mass with no clearly defined structure except a little root-like projection, 

 which extends into the body of the host to absorb the fluids. The em- 

 bryo of Sacculina, however, is a very definitely shaped three-cornered 

 little organism with jointed legs and all other necessary features which 

 bring it under the crustacean classification. In fact, it is practically a 

 degenerated barnacle. 



(b) All higher forms of vertebrates possess so-called gill-pouches 

 during the embryonic stage, although the higher forms do not retain 

 them in the adult stage. This would lead to the assumption that the 

 common ancestors of vertebrates must have been fish-like. 



(c) According to von Baer and Haeckel, all animals during the em- 

 bryonic period pass through the adult forms of the race to which they 

 belong, thus presenting conclusive evidence of the history of their de- 

 scent. 



5. Comparative Physiology. 



Animals which are closely related genetically have a somewhat sim- 

 ilar blood-composition, as proved by the fact that the blood of one such 

 related animal can be successfully transfused to another without harm. 



6. Geographical Distribution. 



Animals such as marsupials (pouched animals) which have as much 

 in common structurally as the Australian kangaroo and the American 

 opossum, while yet quite unlike in general appearance, can only be ac- 

 counted for by taking the geological evidence for a land-bridge into con- 

 sideration which once connected Australia and America. The two ani- 

 mals having had the same ancestry, changed their appearance because of 

 a changed geographical environment, although their general structure has 

 remained quite as it was. 



There are no native ungulates in Australia, although there is no rea- 

 son why there should not be if other than evolutionary methods have 

 been factors in producing new types of animals. 



Or, again, one finds, for example, on the west coast of South America, 

 peculiar animals found nowhere else in the world, while on the neigh- 



