ii2 SCIENCE PROGRESS. 



mann, for one, has thoroughly understood the distinction, 1 

 but as he is more concerned with the establishment of his 

 own evolutionary theory than with the definition and limi- 

 tation of epigenesis as a statement of observed fact, there 

 is danger that the passages in which he makes the dis- 

 tinction will be overlooked, and that his deservedly great 

 authority will be made use of by those who wish to relegate 

 epigenesis to the liinbus fatiiorum. There must be no 

 mistake, epigenesis is a fact, not a theory ; it is only when we 

 attempt to analyse the phenomena of development still 

 further, to go behind the appearances which are familiar to 

 all those who have ever studied the embryology of a few 

 multicellular organisms, and to seek for some explanation of 

 appearances and sequences of which we recognise the form 

 but do not understand the essence, that we are thrown back 

 on a theory which in some measure resembles that of the 

 physiologists of the eighteenth century. Bonnet was per- 

 fectly right when he cast ridicule upon those who asserted 

 that things which could not be seen could not therefore 

 exist. This is admitted at once by Wolff in the passage 

 quoted above, but Wolff was not without reason when he 

 said that this principle had more elegance than truth in it, 

 when applied to the phenomena under consideration. The 

 existence of material particles having form and figure, which 

 are nevertheless beyond the ken of our vision, even when 

 aided by the very best microscopes, can neither be affirmed 

 nor denied by the morphologist. That there are such things 

 as ultimate vital units of which many millions may be re- 



1 " If, however, the id has a right and left half in bilateral animals, 

 we must not thereby infer that it is merely a miniature of the fully formed 

 animal, and that therefore we are once more dealing with the old theory 

 of preformation. Quite apart from all conjectures as to the detailed 

 architecture of the id of germ plasm ; it is at any rate certain that the 

 arrangement of the determinants is quite different from that of the cor- 

 responding parts in the fully formed organism. This is proved by a 

 study of development, and need not detain us here. Any one with a 

 knowledge of animal embryology knows how great a difference there is 

 between the mode of development of the parts from one another in the 

 embryo, and their respective relation in the mature organism " (pp. at., 

 P- 6 5). 



