12 BIOLOGY AND ITS MAKERS 



in this chapter attention will be confined to his observation 

 of their structure and development and to the general in- 

 fluence of his work. 



His great strength was in a philosophical treatment of 

 the structure and development of animals. Professor Osborn 

 in his interesting book, From the Greeks to Darwin, shows 

 that Aristotle had thought out the essential features of 

 evolution as a process in nature. He believed in a complete 

 gradation from the lowest organisms to the highest, and that 

 man is the highest point of one long and continuous ascent. 



His Extensive Knowledge of Animals. He made exten- 

 sive studies of life histories. He knew that drone bees 

 develop without previous fertilization of the eggs (by par- 

 thenogenesis) ; that in the squid the yolk sac of the embryo 

 is carried in front of the mouth; that some sharks develop 

 within the egg-tube of the mother, and in some species have 

 a rudimentary blood -connection resembling the placenta of 

 mammals. He had followed day bv day the changes in the 



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chick within the hen's egg, and observed the development of 

 many other animals. In embryology also, he anticipated 

 Harvey in appreciating the true nature of development as 

 a process of gradual building, and not as the mere expansion 

 of a previously formed germ. This doctrine, which is known 

 under the name of epigenesis, was, as we shall see later, 

 hotly contested in the eighteenth century, and has a modified 

 application at the present time. 



In reference to the structure of animals he had described 

 the tissues, and in a rude way analyzed the organs into their 

 component parts. It is known, furthermore, that he prepared 

 plates of anatomical figures, but, unfortunately, these have 

 been lost. 



In estimating the contributions of ancient writers to 

 science, it must be remembered that we have but fragments 

 of their works to examine. It is, moreover, doubtful whether 



