886 THE THEORIES OF EVOLUTION 



the least specialised, which give rise to new types. The 

 conclusion is that a very high degree of differentiation 

 would preclude the appearance of new variations and 

 that, generally speaking, variability must be on the 

 wane in the organic world. 



Such is in fact the opinion of Rosa and of some 

 other biologists. The question is very complex, for 

 we have no set standard by which to measure the plas- 

 ticity of organisms. We could remark that lower 

 organisms do not die off after giving birth to higher 

 organisms and that nothing prevents them, therefore, 

 from producing new types. It is possible, on the 

 other hand, that lower organisms have become differ- 

 entiated enough to lose their plasticity, for differentia- 

 tion and plasticity are not synonymous, and it would 

 not be unreasonable to assume that a Vorticella, for 

 example, might be as completely differentiated in 

 spite of its simple structure as any of the higher 

 organisms. This would explain why nearly all 

 the main animal and vegetable groups arose in the 

 earliest ages and why no new groups have appeared 

 since. 



What conclusion can we draw from our review of 

 the various biological systems? While none of them 

 offers a satisfactory solution of the problem of evolu- 

 tion, it is undeniable that the various factors of evolu- 

 tion which their authors mention play an important 



