634 ZOOLOGY SECT. 



such as are swiftest, let us say, in escaping pursuit ; or such as, 

 by their special shade of colour or the nature of their markings, 

 elude the observation of an enemy ; or such as, by reason of their 

 thicker covering, can better endure extremes of cold. Such 

 surviving individuals would, it is assumed, transmit their special 

 properties to their progeny, and there would thus be a gradual 

 approximation towards a better adaptation of the species to its 

 surrounding conditions by virtue of this " survival of the fittest." 



Let us suppose the conditions to change. Gradual changes in 

 climate and other conditions are known to take place owing to 

 subsidence or elevation of the land. But conditions might be 

 changed in many other ways : some animal or plant previously 

 used as food might become exterminated ; or a new enemy might 

 find its way into the district inhabited by the species. Then such 

 individuals as presented variations which enabled them better to 

 cope with the new surroundings would have the advantage over 

 the others, and would have a much better chance of surviving and 

 leaving progeny. The useful variations thus produced and trans- 

 mitted to the progeny would tend to increase, generation after 

 generation, until a form sufficiently distinct to be regarded as a new 

 species had become developed from the original one. 



The process of survival of the fittest has a reverse side, which 

 has been termed the elimination of the unfit. Of the varieties 

 that appear some are less completely adapted to their surroundings 

 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, interbreed 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 

 results, 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 



