VI EVOLUTION IN BIOLOGY 223 



valid explanation of the occurrence of varieties and 

 races ; and they saw clearly that, if the explanation 

 would apply to species, it would not only solve the 

 problem of their evolution, but that it would ac- 

 count for the facts of teleology, as well as for those 

 of morphology ; and for the persistence of some 

 forms of life unchanged through long epochs of 

 time, while others undergo comparatively rapid 

 metamorphosis. 



How far " natural selection " suffices for the pro- 

 duction of species remains to be seen. Few can 

 doubt that, if not the whole cause, it is a very im- 

 portant factor in that operation ; and that it must 

 play a great part in the sorting out of varieties 

 into those which are transitory and those which 

 are permanent. 



But the causes and conditions of variation have 

 yet to be thoroughly explored ; and the importance 

 of natural selection will not be impaired, even if 

 further inquiries should prove that variability 

 is definite, and is determined in certain directions 

 rather than in others, by conditions inherent in 

 that which varies. It is quite conceivable that 

 every species tends to produce varieties of a 

 limited number and kind, and that the effect of 

 natural selection is to favour the development of 

 some of these, while it opposes the development 

 of others along their predetermined lines of modi- 

 fication. 



7. No truths brought to ligfit by biological 



