No. 480] 



EXTINCTION OF MAMMMJA 



853 



I believe, been adduced by Mr. Darwin or l)y any other writer 

 on the subject. It is dependent on the fact, that large animals 

 as compared with small ones are almost invariably slow breeders, 

 and as they also necessarily exist in much smaller mnnbers in a 



than do smalh'i' animals. In such an extreme ca-^e a> that of 



are probably as some millions to one; and it is very easily conceiv- 

 able that in a thousand years the former mij^ht, under pressure of 

 rapidly changing conditions, become modified into a distinct spe- 

 cies, while the latter, not offering enough favourable variations to 

 effect a suitable adaptation, would become extinct."^ 



Mr. C. W. Andrews^ has recently (1903) revived this argument 

 that the lengthening of the time taken to attain sexual maturity 

 may affect the rate of evolution, and under changed conditions 



"....In^many Ungulates this increased longevity is i.idieated 

 by various modifications of the teeth, tending to give them a longer 

 period of wear: generally this end is attained by the increasing 

 hypselodonty of the cheek-teeth. A necessary consequence of 

 the longer in<livi<lual life will be that in a given period fewer 

 generations will suc<-eed one an.)ther, and the rate of evolution 

 of th(^ stock will therefon^ be lowtMv.l in the same proportion. If 

 now the conditions of life nn.lergo <-hange, the .,uestion whether 

 a giveti gn)up of am-mals will survive or become extin.-t will depen.l 



it\o avoid getting so far ..ut of harmony 'with its surrounding, 

 that further existence becomes impossible. It seems to follow then 



another rapidly, w^ill have a i)etter clianc(> of surviving than the 



win be still further handi. appe<l if. as i. u>ua!lv xW ca.e, rhey are 

 more hiu-hlv speci:.li/:e<l than th.- smallca- form., and therefore have 

 a more restricted range of possible variation."' 



' ^\■:lll,•H■<^ A. 1!. ( n^ini i>h,r,il I )i st nhut in,, .\„,,„„!s. I. |,p. I r.s l.V.l. 



vol. 10. no. 463, January. IVHKi, p. 2. 



