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 Daruin, shows 
that Aristotle had thought out the essential features of 
cyolution 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 by day the changes in the 
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 
