GENEEAL SUMMARY. 187 



the initial or reproductive cells of the individual. 

 We may suppose a species constant until some dis- 

 turbance of forces affect the very unstable equilibrium 

 of an organism. This would produce variation. 



Physicists have made us aware of some of the 

 phenomena of tension connected with liquid surfaces 

 like those of the soap-bubble when exposed to media 

 of different densities. The walls of a cell, or series of 

 cells, might be similarly acted upon by media of many 

 temperatures, by electrical conditions, and other forces; 

 and such one or more cells would be differentiated 

 from others not so exposed to the like strain. New 

 functions might arise with their concomitant changes. 

 Though the above can be no explanation, it might 

 indicate one of the proximate conditions of vari- 

 ation. 



It has been nowhere proved that the Aristotelian 

 idea of a vital principle, as something either super- 

 added or something distinct from matter, is an erro- 

 neous idea ; or that his opinion and that of his school 

 is illogical, or that the actuality of the intellect has 

 nothing in common with corporeal actuality. 



" Kein Phosphor, Keiu Gredanke " is rich in un- 

 proved assertion. 



Profs. Agassiz and Quatrefages believed. in the immu- 

 tability of species. The former, like Charles Darwin, 

 thought that the question of crossing and infertility 

 had no bearing on the rise of species. Prof. Quatre- 

 fages holds that the distinctive phenomena between 

 race and species furnish the means of recognizing 

 between them ; for, whilst the fertility, intei- se, of the 

 former is undoubted, the persistence of hybridism 

 through crossing, inter se, has never been proved. 



Without wishing to enter on debateable ground, the 

 author thinks that the facts of fertility and infertility 

 ought to have a chief position in the determination of 

 species. It is obvious that persistent infertility must 

 eliminate one or both elements of doubtful forms, and 

 eventually prove such to be mere " sports " in breeding ; 



