Il8 THE HISTORY OF BIOLOGY 



he believes that he has discovered such unity in the egg, out of which all 

 living creatures are evolved. His dictum: "All animals, even those that pro- 

 duce their young alive, including man himself, are evolved out of the egg" 

 is well known. He was naturally not able to observe the eggs of mammals — 

 such a study requires a microscope, which he did not possess — but he pre- 

 supposes their existence on theoretical grounds, a conclusion which was 

 confirmed long afterwards. Nevertheless, he is loath to abandon the idea 

 of primal generation, though he limits this principle to the lowest animals. 

 Out of the egg the higher animals are evolved by epigenesis, in that the 

 organs are successively formed out of the indifferent matter in the egg, which 

 thus constitutes, in harmony with Aristotle's theory, the potentiality out of 

 which the individual is realized. The lower animals, on the other hand, are 

 evolved by metamorphosis, a direct reconstruction of complete rudiments, 

 as is proved especially by the evolution of the pupa of insects; Harvey, in 

 fact, shares Aristotle's belief that the pupa is the insect's egg. On the sub- 

 ject of reproduction his ideas are entirely mediaeval; the influence of the 

 sperm on the development of the embryo he believes to be due to the vital 

 force it contains, and this he compares with the secret force exerted by the 

 heavenly bodies upon all life on the earth. That this last work of Harvey's 

 should also contain a mass of remarkable detailed observations is not sur- 

 prising; he describes with unprecedented care the ovary of the hen and its 

 development, the nourishing of the chicken in the egg, and its growth from 

 the very earliest stages; and of even greater interest are the comparisons he 

 makes between the embryonic stages in different animals — mammals, birds, 

 and lower types. 



Harvey is without doubt one of the most remarkable figures in the his- 

 tory of human culture. His work is the most revolutionary that the develop- 

 ment of biology has to show, for it undermines the foundations of the ancient 

 conception of life and its manifestations, and nevertheless he himself retains 

 this very conception as long as he lives. He thus brings to a close the great 

 epoch in the history of biology which is governed by the ancient conception 

 of nature and he initiates the modern development in the sphere of biology, 

 just as Galileo does in that of physics. How an entirely new science of life 

 has developed on the foundations laid by Harvey will be shown in the next 

 section of this work. 



