EXTENSION OF EVOLUTIONARY THEORIES 331 



Up to the present we have only seen a preparatory 

 principle in isolation, which makes possible the formation 

 of new species. This formation may be brought about 

 by natural selection after the isolation. 



There are also cases in which isolation alone may lead 

 to the formation of a new species. If a fertilised female is 

 forced into an isolated district and lays her eggs there, 

 the young developed from them will not, as a rule, differ 

 from the parental species. But if the female happens 

 to have some peculiar characteristic, it will transmit 

 it to the offspring ; in time the feature will become a 

 characteristic of the isolated animals, as it can no longer 

 be absorbed by crossing. In this way isolation may 

 lead to the formation of a new species without the aid 

 of natural selection. The isolated variety would be still 

 more strongly modified if the characteristic in question 

 were to be accentuated in the offspring ; in other words, 

 if there were some force in the organisms that caused 

 variations to advance in a definite direction in the 

 offspring. We have already found several times that 

 the transformation of species would be brought about 

 much more speedily by a principle of this kind than 

 without it, and we will now inquire whether or no we 

 may infer the existence of such a force. 



We have spoken of selective value in the seventh 

 chapter and seen that it is difficult to conceive how 

 every little variation can be helpful to its possessor in 

 the struggle for life. If that is not the case, how can 

 the variation be preserved and increased ? How could 

 the elephant's trunk be developed from the original nose, 



