16 INTRODUCTORY I 



in 1774 of Caspar Friedrich Wolff's T/ieoria Generations that 

 the evolutionists were aroused from their dogmatic slumbers. 

 Putting speculation on one side, Wolff returned to the method 

 of Harvey, Fabricius, and may we not say also of Aristotle, the 

 method of exact observation. He demonstrated the presence in 

 the unincubated egg not of a complete organism, but of 'globules'; 

 ' partes enim constitutivae, ex quibus omnes corporis animalis 

 partes in primis initiis componuntur, sunt globuli,' l and described 

 the epigenetic formation of the heart and blood-vessels, the 

 central nervous system, the limbs and the ' Wolffian ' bodies from 

 these primary elements. 



Development thus consists of the gradual production and 

 organization of parts; 'embryonis partes sensim produci, mea 

 observata suadent/ 2 and again, ' suppeditari prius partem, deinde 

 earn organisari intelligitur '. 3 



The ground was thus taken from beneath the feet of the 

 preformationists, and Epigenesis restored to its former place of 

 honour as the fundamental expression of developmental fact. 



Tacitly accepted by all the great embryologists of the nine- 

 teenth century Pander, von Baer, Reichert, Bischoff, Remak, 

 Kolliker, Kowalewsky, Haeckel the epigenetic idea continued 

 to control the progress of research. These were men who set 

 themselves to describe the sequence of changes that the embryo 

 passes through with all possible accuracy, and over as wide 

 a range as might be of animal form. They made Comparative 

 Embryology. On the facts that they discovered new light was 

 shed by the doctrine of descent with modification, or evolution 

 in the wider sense of the word. Von Baer had pointed out 

 that in any group of animals the embryos were more like one 

 another than were the adult organisms, and this now became 

 easily translated by Haeckel into the idea that the form which 

 is in every group ultimately in all groups the common starting- 

 point of individual development is representative of the common 

 ancestor of the race. Ontogeny was thus not merely expressed 

 but explained in phylogenetic terms. 



Now that, as we have already seen, the proposed explanation 



1 Wolff, 1. c., Praemonenda, Ixviii. 2 Id. ib. Ixxiii. 



3 Id. ib., De Gen. An., 240. 



