200 EVOLUTION IN BIOLOGY vi 



still less does it consist of the " molecules or- 

 ganiques " 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 con- 

 tains neither in form, nor in substance, more than 

 an inappreciable fraction of the constituents of 

 the germ, and that it is almost, if not wholly, 

 made up of assimilated and metamorphosed 

 nutriment. In the great majority of cases, at 

 any rate, the full-grown organism becomes what 

 it is by the absorption of not-living matter, and 

 its conversion into living matter of a specific type. 

 As Harvey says (Ex. 45), all parts of the body 

 are nourished " ab eodem succo alibili, aliter 

 aliterque cambiato," " ut plantao omnes ex eodem 

 communi nutrimento (sive rore seu terras 

 humore)." 



In all animals and plants above the lowest the 

 germ is a nucleated cell, using that term in its 

 broadest sense ; and the first step in the process 

 of the evolution of the individual is the division 

 of this cell into two or more portions. The pro- 

 cess of division is repeated, until the organism, 

 from being unicellular, becomes multicellular. 

 The single cell becomes a cell-aggregate ; and it 

 is to the growth and metamorphosis of the cells 



