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 decidetl 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. 
