462 Proceedings of the Royal Physical Society. 



can take place amongst living forms that are dominant, 

 without leading to their extinction, is a matter of common 

 observation. However, I think it is still a question for 

 controversy whether these environmental causes are quite 

 sufficient to account for the wliolesale disappearance of large 

 and prominent groups which had become dominant over wide 

 areas of the earth's crust in past geological history. I refer 

 to such groups as are shown on the diagram. Zoologists who 

 have made special study of the geographical distribution of 

 organisms, are tending towards the opinion that animals and 

 plants are much more plastic to changes in their physical 

 surroundings than was formerly thought. They have put 

 forward evidence that barriers to migration are a most 

 important element in the distribution of faunas and floras, 

 both in the past and at the present. It is highly probable 

 that much extinction has been brought about by the breaking 

 down of such barriers to migration in the past with the 

 mixing of two faunas and floras which were originally 

 isolated from one another. This, however, if sudden, can 

 only have acted locally, and any cases involving wide areas 

 will have required geological time; and the whole is involved 

 in the question as to the relation that extinction bears to the 

 struggle for existence. The study of evolution and geological 

 history teaches us that organisms can slowly change from 

 a marine to a land or fresh-water habitat, or again from a 

 littoral or land to an oceanic existence. These developments 

 involve geological time, and more or less alteration in the 

 anatomical structure of the organisms. The power of 

 adaptation to new surroundings, through variation, in some 

 of the lower forms of life, seems indeed almost unlimited. 

 We are not entirely without evidence, moreover, that such 

 changes do occur in some degree with wonderful rapidity 

 amongst forms in the present fauna. Any suddenly- 

 produced changes in the physical surroundings, however, 

 can scarcely act more than locally, and, indeed, all ideas of 

 sudden general extinction of forms of life in the past have 

 been put aside as untenable, since they are not in accordance 

 with tlie well-founded opinions in geology or in the evolution 

 of organisms. To the slowly-produced physical changes, on 



