618 ZOOLOGY SECT. 



than the majority, and these (the conditions remaining the same) 

 tend to become destroyed owing to their unfitness to cope with 

 their environment. The result of this process of elimination 

 (apart altogether from the selection of progressive variations by 

 which evolution, according to the theory, proceeds) is to keep up 

 a certain standard of efficiency in the organs of the members of the 

 species. Under certain conditions this sustaining influence, as we 

 may term it, of natural selection may be suspended ; the organism 

 may be placed under conditions in which natural selection acts with 

 reduced effect or does not act at all. There is, under such circum- 

 stances, no " elimination of the unfit " ; and, as a result, fit and 

 unfit survive indiscriminately, inter-breed, and produce offspring, 

 the ultimate outcome in the course of generations being a gradual 

 deterioration in the whole race. 



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 ; and 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 a positive reversal 

 of selection. 



A special phase of Natural Selection is distinguished under the 

 title of Sexual Selection. By means of Sexual Selection it is 

 attempted to explain the greater part of the secondary differences 

 between the sexes which are so striking in many groups of animals. 

 The special part which each sex has to play in the fertilising and 

 deposition of the ova, in protecting and procuring food for the 

 young, requires qualities, both anatomical and psychical, of a more 



