INTRODUCTORY 23 



subordinate part in the other sciences that it falls to him to assert 

 their importance, since they are so little regarded outside his circle 

 that students in other lines often fail to catch what he has in mind. 

 Among these are the principle of genetic continuity and the prin- 

 ciple of fitness, with all that they imply. For all I know to the 

 contrary, the principle of fitness may be universal, and the order 

 of nature may be the order of fitness ; and again, for all I know to 

 the contrary, all significant resemblances between the phenomena of 

 nature may be due to genetic continuity ; but, at the present day, 

 these principles hold no prominent place in the minds of those who 

 deal with the not-living, and their introduction among the principles 

 of science is due to the biologists. Now only a moment's thought 

 is needed to discover how great are the difficulties that attend the 

 application of these principles. What do we mean by the genetic 

 continuity of life ? How are we to interpret the facts of embryology ? 

 How many perplexing intricacies face us if we undertake, with 

 William Harvey, "to seek the truth regarding the following difficult 

 questions : Which and what principle is it whence motion and 

 generation proceed ? Whether is that which in the egg is cause, 

 artificer, and principle of generation, and of all the vital and 

 vegetative operations, conservation, nutrition, growth, innate or 

 superadded ? and whether does it inhere primarily, of itself, and as 

 a kind of nature, or intervene by accident, as a physician in curing 

 disease ? Whether is that which transfers an egg into a pullet 

 inherent or acquired ? " 



"In truth," says Harvey, "there is no proposition more mag- 

 nificent to investigate or more useful to ascertain than this : How 

 are all things formed by an univocal agent ? How does the like 

 ever generate its like ? Why may not the thoughts, opinions, and 

 manners now prevalent, many years hence return again, after an 

 intermediate period of neglect ? " 1 



As we find embryologists, two hundred and fifty years aft'er these 

 words were written, still vexing themselves over the question, 

 Whether is that which transfers an egg into a pullet inherent or 

 acquired? it is clear that we cannot hope for much progress in the 

 investigation of this magnificent proposition unless we can deter- 

 mine what we mean by that metaphysical notion, inherent potency. 



1 Harvey, " De Generatione," pp. 274-582. 



