XI.] EVOLUTION IN BIOLOGY. 307 



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 modification. 



7. No truths brought to light by biological invest- 

 igation were better calculated to inspire distrust of 

 the dogmas intruded upon science in the name of 

 theology, than those which relate to the distribution 

 of animals and plants on the surface of the earth. 

 Very skilful accommodation was needful, if the limita- 

 tion of sloths to South America, and of the ornitho- 

 rhynchus to Australia, was to be reconciled with the 

 literal interpretation of the history of the deluge ; and, 

 with the establishment of the existence of distinct 

 provinces of distribution, any serious belief in the 

 peopling of the world by migration from Mount Ararat 

 came to an end. 



Under these circumstances, only one alternative 

 was left for those who denied the occurrence of evolu- 

 tion namely, the supposition that the characteristic 

 animals and plants of each great province were 



