MOMENTUM IN EVOLUTION. 277 



Momentum in Evolution. 

 By Professor Arthur Dendy, D.Sc, F.R.S. 



[Ordered by the General Committee to be printed in extenso.] 



It is a fact well known to palaeontologists that many widely separated 

 groups of the animal kingdom have, during the course of their evolution, 

 and especially towards the end of that course shown a strongly marked 

 tendency to enormous increase in size. 1 We see this in the extinct 

 eurypterids, giants amongst the arthropoda, in the huge labyrinthodont 

 amphibians, in many reptiles of the secondary period, some of which 

 attained a length of 180 feet or more, and amongst mammals in the 

 extinct Tinoceras and the still surviving elephants and whales. 



Comparative anatomists are familiar with similar phenomena ex- 

 hibited by individual organs, such as the extraordinary development 

 of horns and spines on many of the extinct reptiles referred to, the 

 gigantic and grotesque beak and helmet of the hornbill and the tusks 

 of Babirusa. 2 



The exuberant development of some organs of this kind may pos- 

 sibly be attributed to the action of sexual selection, and indeed our 

 daily experience of our own species seems to warrant us in believing 

 that there is no limit to the grotesque results which may ensue from 

 the unrestricted exercise of the aesthetic faculties by either sex, but 

 it hardly seems reasonable to attempt to explain all such bizarre and 

 monstrous productions in this manner. 



In all the cases cited, and in many others which could be adduced, 

 either the entire body or some particular organ appears to have acquired 

 some sort of momentum, by virtue of which it continues to grow far 

 beyond the limits of utility, although perhaps in some cases a new 

 use may be found which will assist the species in maintaining itself 

 in the struggle for existence. An enormous increase of mere bodily 

 size, however, seems in the long run to be always fatal to the race, 

 whose place will be taken by smaller and presumably more active forms. 

 The gigantic amphibians are all extinct, so are all the really gigantic 

 reptiles, and of the gigantic mammals only a couple of species of 

 elephants and a few whales survive, all of which are being rapidly 

 exterminated in competition with man. 



1 Vide Dr. Smith Woodward's Presidential Address to the Geological Section of 

 the British Association, 1909. 



2 Darwin supposed that these tusks, which arc curved backwards in such a posi- 

 tion as to render them useless as weapons of offence, might still be defensive and 

 used to parry blows, but this hardly seems a sufficient explanation of their enormous 

 development. 



