XL] EVOLUTION IN BIOLOGY. 285 



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 

 step, from an extreme simplicity, or relative homo- 

 geneity, 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, however, 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 

 elementary constituents of the germ, as Bonnet im- 

 agined; still less does it consist of the "molecules 

 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 

 the living body into which it enters. And, so far 

 from the fully developed organism being simply the 

 germ plus the nutriment which it has absorbed, it is 

 probable that the adult contains neither in form, nor in 

 substance, more than an inappreciable fraction of the 



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

 gate if we call germ only that which is already detached from the 

 parent organism. 



