xv THE PHILOSOPHY OF ZOOLOGY 655 



This suspension of the influence of natural selection, with its re- 

 sults, has been termed cessation of selection, or panmixia. Panmixia 

 acts more commonly on single organs than on the entire organism. 

 Thus, if, owing to some change in surrounding conditions, an organ 

 is no longer useful, it is no longer kept up to the previous degree of 

 efficiency by the elimination of the individuals in which the organ 

 in question is imperfectly developed, and, as these cross with one 

 another, offspring is produced in which the organ is below the 

 efficient standard ; by a continuance of this process through a 

 series of generations, it is supposed that the organ gradually 

 dwindles in size, and may altogether disappear. Thus at that 

 stage in the ancestral history of the Cetacea in which they had 

 come to adopt a purely aquatic mode of life and no longer visited 

 the shore, the hind-limbs, being no longer of service, would no 

 longer be maintained by natural selection, and would gradually 

 decrease in size until, finally, they entirely disappeared. In the 

 case of these, as of many other rudimentary organs, however, it is 

 probable that natural selection played a positive part in bringing 

 about their diminution. Under the conditions supposed, the 

 possession of hind-limbs would probably be an actual disadvantage 

 to the animal, acting as an impediment to the swift progression 

 through the water, and interfering with the free movements of the 

 tail ; and varieties with diminished hind-limbs would, therefore, 

 possess an advantage over their fellows in the struggle for existence. 

 There would then be, in a sense, a positive reversal of selection. 



Darwin's theory of selection is concerned mainly with the small 

 individual variations which are observed to occur, more frequently 

 in some species, more rarely in others. Such variations are so 

 slight and unimportant that it is difficult to understand how 

 they could be of sufficient life-and-death value to give the indivi- 

 duals in which they occur sufficient advantage in the struggle for 

 existence to enable them to survive, when others in which they are 

 absent perish. Failing the extermination of the unmodified 

 individuals, unless the appearance of the variation should happen 

 to be coincident with the occurrence of other factors leading to 

 the isolation of the individuals possessing the new variation from 

 the stock in which they originated, the new variety would tend 

 to become swamped by inter-crossing with the latter and would 

 fail to be perpetuated. If, however, the individuals in which the 

 new modification occurs should by some means such as migration 

 beyond a geographical barrier of some kind, or by the nature 

 of the variation itself be preserved from inter-crossing with 

 the stock, then, without the extermination of the latter being 

 a necessary condition without, that is to say, a life-and-death 

 struggle, the new form might be preserved unaltered and perpetu- 

 ated as a new and distinct variety, which further changes similarly 

 brought about might raise to the rank of a species. 



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