A Stcggestion on Extinction. 463 



the other hand, which have acted over wide areas during 

 geological history, the degree of plasticity of organisms seems 

 quite capable of answering. The great degree of this plas- 

 ticity to the surroundings, through variation, which occurs 

 within the limits of closely -related groups, is demonstrated 

 by the very wide variation in the habits and modes of exist- 

 ence of forms which have undoubtedly sprung from a near 

 and common ancestor. Such considerations as the above, 

 and many more which might be added, would suggest that 

 the changes in the physical conditions of the past have not 

 been the sole cause of the extinction of large groups of animals 

 and plants which had become dominant and widespread. 



The view regarding extinction, which considers the mutual 

 relations which exist between the different individuals and 

 groups of animals and plants to their environment, is the one 

 which was so ably advocated by Darwin in his " Origin of 

 Species," and therefore requires the closest consideration. 

 He pointed out that the destruction of life which ensues 

 from the struggle for existence, and consequent Natural 

 Selection, would be sufficient to account for tlie constant 

 extinction in the fauna and flora of the past. He, in fact, 

 goes so far as to say that it would be folly to wonder at 

 extinction when we consider the enormous destruction that 

 must take place amongst the forms that were not successful 

 in the struggle for existence. I need hardly say that the 

 very large part which is taken by Natural Selection in sur- 

 vival and extinction throughout all forms of life is only too 

 apparent, and its demonstration must be thoroughly satisfac- 

 tory to all who believe in evolution. This would be the sole 

 necessary explanation for the extinction of groups in the past, 

 provided we were quite sure that there was not some factor 

 coming from within the organism which tends in time to 

 cause a weakness of the individual in relation to its environ- 

 ment or an increasing want of fertility. Should such be the 

 case, the often-repeated suggestion of the likeness of the life- 

 history of a group in time with its periods of onset, domin- 

 ance, degeneration, and extinction, to the life and death of 

 the individual, would be indeed more than a mere simile. 

 The study of the ranges in time of the extinct groups in the 



