264 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



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 which it exists : it cannot have arisen through an 

 internal developmental force. But if single species, or even whole 

 orders like the Ceiacea, 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 trans- 

 formation 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 in- 

 compatible with certain important points in his general theory, and 

 which therefore necessitate some modification 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 con- 

 ditions of life to which an organism must adapt itself if it is to 

 continue in existence. Thus a selective process is set up which 

 ensures that only those out of the existing variations are pre- 

 served, which correspond in the highest degree to the changed 

 conditions of life. By continued selection in the same direction 

 the deviations from the type, although at first very insignificant, 

 are accumulated and increased until they become specific differ- 

 ences. 



I should wish to assert more definitely than Darwin has done, 

 that alterations in the conditions of life, together with changes in 

 the organism itself, must have advanced veiy gradually and by the 

 smallest steps, in such a way that, at each period in the whole pro- 

 cess of transformation, the species has remained sufficiently adapted 

 to the surrounding conditions. An abrupt transformation of a species 

 is inconceivable, because it would render the species incapable of 

 existence. If the whole organization of an animal depends upon 

 adaptation, if the animal body is, as it were, an extremely complex 

 combination of new and old adaptations, it would be a highly 

 remarkable coincidence if, after any sudden alteration occurring 

 simultaneously in many parts of the body, all these parts were 



