vi EVOLUTION IN" BIOLOGY 199 



gated, the substance of this germ has a peculiar 

 chemical composition, consisting of at fewest four 

 elementary bodies, viz., carbon, hydrogen, oxygen, 

 and nitrogen, united into the ill-defined compound 

 known as protein, arid associated with much 

 water, and very generally, if not always, with 

 sulphur and phosphorus in minute proportions. 

 Moreover, up to the present time, protein is known 

 only as a product and constituent of living 

 matter. Again, a true germ is either devoid of 

 any structure discernible by optical means, or, at 

 most, it is a simple nucleated cell. 1 



In all cases the process of evolution consists in 

 a succession of changes of the form, structure, 

 and functions of the germ, by which it passes, 

 step by st^p, from an extreme simplicity, or rela- 

 tive homogeneity, of visible structure, to a greater 

 or less degree of complexity or heterogeneity; 

 and the course of progressive differentiation is 

 usually accompanied by growth, which is effected 

 by intussusception. This intussusception, how- 

 ever, is a very different process from that imagined 

 either by Buffon or by Bonnet. The substance 

 by the addition of which the germ is enlarged is 

 in no case simply absorbed, ready-made, from the 

 not-living world and packed between the elemen- 

 tary constituents of the germ, as Bonnet imagined ; 



1 In some cases of sexless multiplication the germ is a cell- 

 aggregate if we call germ only that which is already detached 

 from the parent organism. 



