184 THE LESSON OF EVOLUTION 



due to inter-breeding, but sometimes a new variation 

 may be prepotent over the old type from which it 

 has but recently arisen. This may account for the 

 two new species of cormorants, which I have just 

 mentioned, swamping the older forms. 



But the most important cause of physiological 

 isolation is change in the time of the maturation of 

 the reproductive cells. These are sometimes acce- 

 lerated, sometimes retarded, so that all the indi- 

 vidual members of a species cannot breed at the same 

 time. This I believe to be a very common cause of 

 isolation in both plants and animals ; and I think that 

 it has been the chief cause of species splitting up 

 into two or more. Its action is evident, so I will 

 give an example of a mixed case, in which physio- 

 logical isolation has led to physical isolation. 



In the southern seas no two species of Albatross 

 breed in the same locality. Even when two dif- 

 ferent kinds are found on the same island as 

 D. exulans and D. regia on Adams Island, one of 

 the Auckland group they occupy widely separated 

 sites ; and they all breed at different times. So far 

 as I know, D. cauta, of the Bounty Islands, is the 

 "first to commence to breed, at the end of August. 

 D. melanophrys, at Campbell Island, comes next, 

 in the middle of September. D. regia, at Campbell 

 Island, commences in the middle of November. 

 D. chionoptera, at Kerguelen, at the middle or end of 

 December. D. exulans the first week in January at 

 Adam's Island, and the middle of January at Anti- 

 podes Island. And last comes D. culminatus, on the 

 Snares, at the end of January. So that there is no 



