xv THE PHILOSOPHY OF ZOOLOGY 635 



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 individuals 

 in which they occur sufficient advantage in the struggle for existence 

 to enable them to survive, w r hen 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 

 intercrossing 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 intercrossing 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 perpetuated as a new and distinct variety, 

 which further changes similarly brought about might raise to the 

 rank of a species. 



Detailed study of the geographical distribution of species and 

 varieties in certain regions, more especially in the United States of 

 America, has afforded much support to the view that the develop- 

 ment of new forms takes place as a result of the appearance of 

 varieties differing slightly from the parent stock and their isolation 

 from the latter by geographical barriers ; and by some writers 

 evolution is even supposed to have proceeded solely, or almost 

 solely, in this way with little, or entirely without, aid from natural 

 selection. Another kind of isolation might be supposed to take 



