292 EVOLUTION IN BIOLOGY. 



position, consisting of at fewest four elementary bodies, 

 viz. carbon, hydrogen, oxygen, and nitrogen, united into 

 the ill-defined compound known as protein, and associ- 

 ated 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.* 



In all cases, the process of evolution consists in a 

 succession - of changes of the form, structure, and func- 

 tions of the germ, by which it passes, step by step, from 

 an extreme simplicity, or relative homogeneity, of visi- 

 ble structure, to a greater or less degree of complexity 

 or heterogeneity; and the course of progressive differ- 

 entiation 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 ab- 

 sorbed ready-made from the not-living world and packed 

 between the elementary constituents of the germ, as 

 Bonnet imagined ; still less does it consist of the " mo- 

 lecules organiques" of Buffon. The new material is, in 

 great measure, not only absorbed but assimilated, so that 

 it becomes part and parcel of the molecular structure of 



* 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 or- 

 ganism. 



