VI DIFFICULTIES AND OBJECTIONS 151 



if this were a vera causa, we should expect to find it. In 

 Ireland we have an excellent test case, for we know that it 

 has been separated from Britain since the end of the glacial 

 epoch, certainly many thousand years. Yet hardly one of 

 its mammals, reptiles, or land molluscs has undergone the 

 slightest change, even although there is certainly a distinct 

 difference in the environment both inorganic and organic. 

 That changes have not occurred through natural selection, is 

 perhaps due to the less severe struggle for existence owing to 

 the smaller number of competing species ; but, if isolation 

 itself were an efficient cause, acting continuously and cumula- 

 tively, it is incredible that a decided change should not have 

 been produced in thousands of years. That no such change has 

 occurred in this, and many other cases of isolation, seems to 

 prove that it is not in itself a cause of modification. 



There yet remain a number of difficulties and objections 

 relating to the question of hybridity, which are so important 

 as to require a separate chapter for their adequate discussion. 



