1895.] NATURAL SCIENCES OF PHILADELPHIA. 253 



THE EXTINCTION OF SPECIES. 

 BY CHARLES MORRIS. 



The story of the disappearance of species of animals and plants 

 during the geological period, and their replacement by new species, 

 is much too obscure to attempt to solve except to a very partial 

 extent. Since life began upon the earth the process of extinction 

 has been seemingly incessant, and still continues so, Man being per- 

 haps the most efficient of all the many agents that have taken part 

 in this process. As a rule, the extinction of forms would appear to 

 have been gradual, and the simultaneous sweeping out of great 

 numbers of species, which is claimed occasionally to have taken 

 place, is open to question, from our lack of knowledge of the 

 length of time really involved. There are, however, two instances 

 in which such an extinction of numerous species within a com- 

 paratively brief interval appears certainly to have taken place, and 

 it is these which I propose particularly to consider. These are the 

 apparently sudden disappearance of great numbers of giant reptilian 

 forms at the close of the Cretaceous epoch, and their replacement in 

 the early Tertiary by numerous large mammals; and the similar 

 sudden disappearance of a considerable number of large mammals in 

 the Post-pliocene or the early Recent period, including the mammoth, 

 the American horse, the giant sloths, etc. Although no other than 

 general causes can be adduced for extinction as a whole, it may be 

 that some particular causes can be suggested in connection with 

 these two special instances. 



As an important preliminary to this investigation, some considera- 

 tion of the general causes of the disappearance of species is desirable. 

 In the first place it is improbable that many, if any, species have 

 ceased to exist in consequence of the direct assaults of other animals, 

 except in the case of Man's destructive agency. A species, whose 

 individuals are numbered by millions and whose range is ordinarily 

 very extended, is not easily to be disposed of. Animal hostility, 

 even when unrelenting and effective, is never governed by a fixed 

 purpose of destruction. Its aims are minor and individual; food, 



