408 General Biology 



3. Comparative Anatomy. 



The similarity of structure in different individuals is precisely what 

 would be experienced if evolution did take place. 



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 years 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 

 embryonic period pass through the adult forms of the race to which 

 they belong, thus presenting conclusive evidence of the history of their 

 descent. 



5. Comparative Physiology. 



(a) Animals, which are closely related genetically, have a some- 

 what similar blood-composition, as proved by the fact that the blood 

 of one such related animal can be successfully transfused to another 

 without harm. 



(b) The test described in Chapter XIV by which human blood 

 can be differentiated from that of many lower mammals, does not differ- 

 entiate human blood from the blood of the higher apes. 



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 

 accounted for by taking into consideration the geological evidence for 



