372 THE EVOLUTION THEORY 



formed themselves into a stable association with definite form and 

 definite structure, somewhat analogous to the spherical cell-colonies 

 of Alagosphceva or Pandorina. Only then was the further step made 

 of a differentiation of the individual biophors forming the colony, and 

 this is comparable to the species of Volvox among the lower Algse. 

 The gradual ascent of these colonies of biophors must, then, be 

 referred to the principles to which we attribute the ascent of the 

 higher forms of life to ever-higher and ever-new differentiations ; 

 the principles of division of labour and selection. 



These differentiated colonies of biophors have brought us nearer 

 to the lowest known organisms, among which there are some whose 

 existence we can only infer from their pathological effects, since 

 we have not been able to make them visible. The bacillus of measles 

 has never yet been seen, but we cannot doubt its existence, and 

 we must assume that there are bacilli of such exceeding smallness 

 that we shall never be able to see them, even with the most improved 

 methods of staining and the strongest lenses. 



These non-nucleated Monera lead on to the stage of nucleus- 

 formation, and this at once implies the cell. As, on our view, the 

 nucleus is primarily a storehouse of 'primary constituents' (Anlagen), 

 its origin must have begun at the moment at which the differentiation 

 of the cell-body reached such a degree of differentiation of its parts 

 that a mechanical division into two halves was no longer possible, 

 and that the two products of division, if they were each to develop 

 to a new and intact whole, required a reserve of primordia (Ardagen) 

 to give rise to the missing parts. As this higher differentiation 

 would bring about a superiority over the lower forms of life, in that 

 they would make possible the utilization of new conditions of life, 

 but on the other hand could only survive if the differentiation of 

 a reserve of primary constituents, that is, a nucleus, were introduced 

 at the same time, the development of the nucleus can be ranged under 

 the principle of utility to which we traced back the evolution of 

 all higher and more differentiated forms of life. But it would 

 scarcely be profitable to try to follow out in detail the first steps 

 in the progress of organization under the control of selective processes, 

 since we know far too little about the life of the simplest organisms 

 to be able to judge how far their differentiations are of use in improv- 

 ing their capacity for life. 



That would be a bold undertaking even in regard to unicellular 

 organisms, and it is only in the case of multicellular organisms that 

 we can speak with greater certainty and really recognize the changing 

 of the external conditions, in the most general and comprehensive 



