506 



THE THEORY OF EVOLUTION 



vidual development, or ontogeny, is not an exact repetition of 

 ancestral development, or phytogeny, but certain structures that 

 are reminiscent of ancestry may be found in embryos. 



In addition to gill slits, many other features in the embryos of 

 amphibians and higher vertebrates are evidently paUngenetic; 

 and similar reminiscences appear in the development of other 

 types of animals. The difficulty is that palingenetic and ceno- 



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j^^-y ji 





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Fig. 269. — Reconstruction of one of the carnivorous dinosaurs, Ceratosaurm, 

 shown feeding with background of vegetation based upon associated fossils 



of plants. 



The dinosaurs were the dominant land animals in the Age of Reptiles (c/. Fig. 259) . 

 but became extinct as the mammals increased in numbers. (From Lucas, "Animals of 

 the Past," courtesy, American Museum of Natural History.) 



genetic features are so intermingled that it is often impossible to 

 distinguish between them. 



Classification. — The Basis of Biological Classification. — Ani- 

 mals and plants may be classified in a variety of ways. If one is 

 interested in Ecology one may group them according to habitat, 

 by separating fresh-water animals from land animals or the animals 

 of forests from those of prairies. Again, they may be classified 

 according to their food, or even according to their size, as one 

 classifies books by putting them in a book-case to suit the shelf 

 room. But just as there is one basis of classification that is the 

 most fundamental for books, namely subject matter, so for living 



