70 DOCTRINE OF EVOLUTION 



up" are explained and understood by the simple for- 

 mula that the mode of individual development owes its 

 nature primarily to the hereditary influence of earlier 

 ancestors back to the original animals which were 

 protozoa. 



Embryology as a distinct division of zoology has grown 

 out of studies of classification and comparative anatomy. 

 Its beginnings may be found in medieval natural his- 

 tory, for as far back as 1651 Harvey had pointed out 

 that all living things originate from somewhat similar 

 germs, the terse dictum being "Ex ovo omnia." By the 

 end of the eighteenth century many had turned to the 

 study of developing organisms, though their views by no 

 means agreed as to the way an adult was related to the 

 egg. Some, like Bonnet, held that the germ was a 

 minute and complete replica of its parent, which simply 

 unfolded and enlarged like a bud to produce a similar 

 organism. Even if this were true, Uttle would be gained, 

 for it would still remain unknown how the germinal 

 miniature originated to be just what it was conceived 

 and assumed to be. Wolff was the originator of the 

 view that is now practically universal among natural- 

 ists, namely, that development is a real process of 

 transformation from simpler to more complex condi- 

 tions. 



The subject of comparative embryology grew rap- 

 idly during the nineteenth century as the field of com- 

 parative anatomy became better known, and when 

 naturalists became interested in animals, not only as 

 specific types, but also as the finished products of an 



