CHAPTER 3 



EVOLUTION AS SEEN IN 



THE STRUCTURE OF MODERN 



ANIMALS 



Morphology 



Structure is the easiest aspect of an animal to study. 

 Perhaps it is for this reason that knowledge of animal structure dates from 

 ancient times and was, indeed, the first aspect of biology to develop. The 

 study of structure is called morphology, a word of slightly broader mean- 

 ing than the more familiar term "anatomy," which is nearly synonymous. 

 Biologists had not progressed far in the study of morphology before they 

 were impressed by similarities among different animals and began to 

 speculate as to the reason for these similarities. 



Analogy 



Why are different animals similar in structure? In the first place, we may 

 note that there is no cause for surprise in the fact that animals living in the 

 same environment or having similar methods of locomotion, obtaining 

 food, and so on, resemble each other. Fishes and whales are both faced 

 with the problem of moving rapidly through water. What could be more 

 natural than that they both should have streamlined body forms and 

 should be propelled by the thrust of powerful tails against the surrounding 

 water? Or again, birds and bats utilize the air as a medium of locomotion. 

 Both, therefore, possess wings which, like the wings of an airplane, sup- 



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