THE EVIDENCE FOR ORGANIC EVOLUTION 



503 



and horses and all other mammals had a common ancestry at a 

 remote period, their fore limbs and many other parts have certain 

 similarities to-day. Mammals resemble other vertebrates for a 

 similar reason. The facts of Anatomy are, therefore, what might 

 be expected if evolution has occurred. 



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Fig. 266. — Representative fossils. 



A, impressions of a leaf and an insect larva. B, a trilobite, Phillipsia griffithiden, repre- 

 sentative of a great class of Arthropoda that flourished in the Pala;ozoic era (r/. Fig. 259) 

 and later became extinct. C, a rich deposit of shells, principally the marine mollusc 

 Astarte. It was such remains as these (O that led Leonardo da Vinci (1452-1519) to 

 conclude that the rocks he observed even in the high mountains of Italy had originated 

 on the ocean's floor and hence that profound evolutionary changes had occurred in the 

 history of the earth. (Photos, by courtesy of the American Museum of Natural History.) 



Comparative Embryology. — Almost everyone who knows any- 

 thing of the evidence for organic evolution has heard the statement 

 that the development of an individual is a repetition of the develop- 

 ment of the race. This " Recapitulation Theory," as it is called, 

 maintains that certain developmental stages or structures are 

 palingenetic, that is, related to ancestral conditions; while others 



