I 7 o CHAPTERS ON EVOLUTION. 



perfectly formed therein. Thus, by Malpighi's view, the process of 

 development was merely one of the expansion, unfolding, and en- 

 largement of parts already formed ; and this idea became known as 

 that of Metamorphosis, in contradistinction to Harvey's theory of 

 " Epigenesis." So, also, Bonnet maintained the existence of a minia- 

 ture chick in the egg from the first moment of its formation. Subse- 

 quent growth and nutrition merely expand the elements and parts of 

 this germ into those of the adult ; and thus Bonnet declares the 

 process of development to be merely one of " Evolution." Thus 

 the doctrine of " Epigenesis," as enunciated by Harvey, becomes 

 opposed to that of " Evolution," as maintained by Bonnet and 

 Haller the development of new parts and structures from a struc- 

 tureless substance, as distinguished from the mere enlargement 

 and unfolding of the miniature but already formed elements of the 

 frame. 



But when Bonnet, in 1762, in his work entitled "Considerations 

 sur les Corps organises," was elaborating his theory of " Evolution " 

 and less rational views on " Emboitement " a theory holding that 

 each germ is the receptacle of the germs of all future beings of its 

 race Caspar Friederich Wolff had already lent his aid towards 

 placing the Harveian views on a secure and stable basis. Wolff 

 showed that the scar on the hen's egg consisted of particles amidst 

 which no rudiment of an embryo chick could be traced. He further 

 demonstrated the changes whereby the chick was built up from these 

 cells, and showed the process of development to be truly one wherein 

 new parts were formed in succession, and added to the already formed 

 organs. Succeeding Wolff came Pander, who filled in the outlines 

 his predecessor had so well sketched out by detailing the earlier 

 stages and processes seen in the formation of the young bird. From 

 Pander came the name blastoderm, given then, as now, to the substance 

 or formative material resulting from early changes in the " egg-scar," 

 and from which material all the parts of the young animal are formed. 

 This observer also cleared the way for his successors by pointing out 

 the presence of the three layers into which the blastoderm divides ; 

 each layer bearing an important share in the formation of the tissues 

 of the developing being. To Pander came in due time a worthy 

 successor, who may be said to have laid the solid foundations of 

 the study of development as prosecuted in modern times. This was 

 Von Bae'r, whose labours each physiologist and naturalist of to-day 

 must hold in grateful remembrance. He it was who, besides per- 

 fecting the details already to hand, discerned the important fact 

 that the highest animals are developed from eggs or germs re- 

 sembling in essential nature those of the lowest. But perhaps the 

 greatest triumph of discovery and research as represented by 

 Von Bae'r's labours resulted in the enunciation of his " law of deve- 



