RECORDS. 369 



also consider briefly their relation to recent conclusions affect- 

 ing our theories of heredity and evolution. 



Let us first seek to define more clearly the meaning of our 

 terms. The embryologists of the pre-Darwinian period, unham- 

 pered by historical conundrums, fixed their attention on the 

 single objective problem of the nature of the germ and its mode 

 of development. The hen's egg contains something which, 

 though not visibly a bird or even an embryo, will when main- 

 tained at a temperature of about 37° C. for 21 days, cause a liv- 

 ing chick to step forth from the shell. What is that something 

 and what manner of machinery (if machinery it be) is set in 

 motion to work such a marvel ? The early embryologists found 

 no real answer to this question. They determined the fact that 

 at the beginning the egg contains nothing even remotely resem- 

 bling a bird ; that as early as the second day a rudely fashioned 

 embryo is visible in the egg : and that day by day, as the incu- 

 bation proceeds, this embryo becomes more complex. The bird 

 appears to be progressively created out of something that is 

 without form, and void of visible structure. Its development, 

 said Harvey and Wolff, is essentially a process of'* epigenesis " — 

 a successive formation and addition of new parts not previously 

 existent as such in the egg. This conclusion, roughly outlined 

 by Aristotle, was apparently established on an irrefragable basis 

 of observation, long afterwards, by Harvey and Wolff. In its 

 superficial aspects the doctrine of epigenesis is no more than a 

 statement of universally admitted fact. When followed to its 

 logical end, however, this conception has failed, and will always 

 continue to fail, to satisfy the mind ; and some of the most acute 

 of modern embryologists have expressed the opinion that no 

 thoroughgoing hypothesis of epigenesis can be so framed as to 

 be logical, or even conceivable. Even in the eighteenth century 

 this doctrine was met by the opposing one of preformation and 

 evolution. Advocated by such men as Malpighi, Haller and 

 Leibnitz, this conception underwent its fullest development in 

 the hands of the eminent Swiss naturalist Bonnet. Developed 

 with great logical acuteness and set forth with captivating liter- 

 ary skill, Bonnet's theory was based on the fundamental assump- 



