270 SIGNIFICANCE 01 SEXUAL REPRODUCTION [V. 



surface, and at that very time in the earth's history, which 

 offered the conditions appropriate for its existence. As I have 

 previously argued, the beHevers in an internal developmental 

 force are compelled to invent an auxiliary hypothesis, a kind 

 of 'pre-established harmony' which explains how it is that 

 changes in the organic world advance step by step, parallel 

 with changes in the crust of the earth and in other conditions 

 of life ; just as, according to Leibnitz, body and soul, although 

 independent of each other, proceed along parallel courses, like 

 two chronometers which keep perfect time. And even this 

 supposition would not be sufficient, because the place must be 

 taken into account as well as the time : thus the whales could 

 not have existed if they had first appeared upon dry land. We 

 know of countless instances in which a species is exclusively 

 and precisely adapted to a certain localized area, and could not 

 thrive anywhere else. We have only to remember the cases 

 of mimicry in which one insect gains protection by resembling 

 another, the cases of protective resemblance to the bark or the 

 leaves of a certain species of plant, or the numerous marvellous 

 adaptations of parasitic animals to certain parts of certain 

 species of hosts. 



A mimetic species cannot have appeared at any place other 

 than that in w^hich it exists : it cannot have arisen through an 

 internal developmental force. But if single species, or even 

 whole orders like the Cetacea, have arisen independently of any 

 such force, then we may safely assert that the existence of the 

 supposed force is neither required by reason nor necessity. 



Hence, abstaining from the invocation of unknown forces, we 

 are justified in carrying on Darwin's attempt to explain the 

 transformation of organisms by the action of known forces and 

 known phenomena. I say ' carry on the attempt,' because I do 

 not believe that our knowledge in this direction has ended with 

 Darwin, and it seems to me that we have already arrived at 

 ideas which are incompatible with certain important points in 

 his general theory, and which therefore necessitate some modi- 

 fication of the latter. 



The theory of natural selection explains the rise of new 

 species by supposing that changes occur, from time to time, in 

 those conditions of life to which an organism must adapt itself 

 if it is to continue in existence. Thus a selective process is set 



