THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. X. 



No. I.— JANUARY, 1903. 



OK-IG-in^r^LL -A-DESTXCXjIES. 



I. — Some Suggestions on Extinction. 



By C. W. Andrews, D.Sc, F.G.S., British Museum (Natural History). 



ri1HE sudden disappearance of groups of animals, which have 

 L existed through long periods of time and have attained a high 

 degree of specialization, is a phenomenon of which many instances 

 will occur to every student of Palseozoology. For instance, to 

 mention only two cases in illustration, we may refer to the disap- 

 pearance of the Dinosaurs at the end of the Secondary period, and 

 that of the North American Titanotheres in the Miocene. Of the 

 proximate causes of this extinction little is known : they must have 

 been either inherent in the organisms themselves, or have been 

 connected with the relations of the organisms with their environ- 

 ment; probably in every case several factors co-operated to bring 

 about the observed result. In a recent paper by Mr. C. B. Crampton 

 (Proc, Eoy. Phys. Soc. Edinburgh, vol. xiv, p. 461) a possible 

 inherent cause of extinction is suggested. It is impossible to do 

 justice to this interesting paper in a short note, but the gist of the 

 argument seems to be as follows : — In the original unicellular 

 oi'ganism the possibilities of variation are almost infinite, but as 

 soon as evolution along any line begins, these possibilities are 

 restricted, and become more and more so the more highly specialized 

 the animal is ; in short, the potential variation of an organism 

 becomes less and less as specialization advances. Furthermore, 

 under the influence of natural selection, in each generation the 

 individuals which tend to vary in the same direction will survive, 

 while at the same time, as already pointed out, their capacity for 

 variation becomes more and more restricted. The consequence of 

 this will be that the more highly specialized any stock becomes, the 

 more the individuals composing it will come to resemble one 

 another, until at length the same results as arise from close inter- 

 breeding, viz., weakening of the stock and, finally, extinction, 

 may follow. 



In a paper recently read before the Zoological Society, the present 

 writer, in speaking of the evolution of the Proboscidea, took the 



DECADE IV. — VOL. X. — NO. I. 1 



