FUOM AN OLD. STANDPOINT. 



187 



Part II. 



ox some of the evidence bearing on the theory of 

 Differentiation. 



The second part of this paper is devoted to a further 

 discussion of the testimony favouring; the differentiation 

 hypothesis. The principle of the concurrent differentiation of 

 plants and their life-conditions is either tacitly assumed or 

 directly implied in the writings of several botanists. Of many 

 again it may be said that whilst "evolution" is always on 

 their banner, " differentiation " is ever on their lips. Indeed it 

 is not easy to take up a book dealing with the development or 

 with the distribution of plants and animals without discovering 

 some pregnant sentence connected with this theory. There are 

 others who state the theory both on its physical and its biological 

 side so aptly and concisely that one wonders how they did not see- 

 their limitations and accept it as a good working hypothesis. 

 Their difB.culty, however, lay in the fact that differentiation acts 

 only within the type, and that if we wish to discover the 

 progressive development of types we must look elsewhere for 

 the causes. The position adopted in this paper is that these 

 causes are hidden from us, and that the only operations evident 

 to us in nature are those concerned with diversification of 

 already existing types. 



The theory here advocated is two-sided. On the one hand 

 there are the differentiating life-conditions which mainly find 

 expression in the diversiKcation of climate, and on the other 

 hand there are the differentiating organisms. The connection, 

 between the organism and its conditions is implied m the 

 prevailing views of adaptation apart from any particular theory, 

 and I need not labour that point. The subject then has its 

 physical and its biological side, and althougii we are here 

 immediately concerned with plants, it should not be forgotten 

 that if tlie hypothesis is a workable one it will apply also to- 

 animals. 



TJlc Physical side of tlic Differentiation Theory. 



Little can be said here of the earliest stage of the conditions 

 of life on the earth, an age when uniformity of conditions pre- 

 vailed, an era, indeed, of Cimmerian gloom, when the sun's rays 

 were screened off by dense envelopes of cloud and mist, 

 and when the air was ever heavy with aqueous vapour. It is 



