50 ORGANIC EVOLUTION — PHYSICAL 



and tliG powers of assimilation, is checked by natural 

 selection, Avhich, by placing at a disadvantage, brings 

 about tlic idtimato elimination of multicellular organ- 

 isms, in whicli the tendency is displayed. 



On the other hand, this or that cell-descendant of 

 the germ may vary from the normal in a direction 

 which is not towards the ancestral type, but in a 

 different direction, and its descendant cells then form 

 a mass or group wliich also differs more or less 

 from the normal. The bud variations which have 

 been observed on several cultivated plants, by taking 

 advantage of which gardeners have sometimes been 

 able to establish new varieties, probably result from 

 this cause. 



Secondly, each successive germ cell must differ from 

 the ancestral conjugating unicellular organism more 

 and more, in that different lines of its cell-descendants 

 differentiate more and more from one another in struc- 

 ture and function, Avhence arise in high animals differ- 

 entiated and specialized tissues such as muscle, bone, 

 skin, gland, nerve, &c. 



Moreover, just as each germ cell during the phylogeny 

 differed more and more from its prototype, the unicellular 

 organism, in that it was the starting-point of a more 

 and more complex organism (mass of cell-descendants), 

 so each embryo during the whole process of its ontogeny 

 differs from its prototypes in the phylogeny, in that it 

 carries within it the potentiality of developing beyond 

 those prototypes. We cannot, however, discern in the 

 macroscopic or microscopic appearances of germ or 

 embryo any peculiarities of structure which imply this 

 potentiality. They lie beyond the ken in the minute 

 structure of the cells, probably in those portions of them 

 which arc known as the nuclei. 



It may be objected, (1) that it is impossible that 

 during the short period of the development of the 



