66 ZOOLOGY 



in greater intensity; and by a continuance of such selective 

 mating a race differing markedly from the parent race was 

 produced. Thus, for instance, two breeders of sheep who had 

 different ideals as to the type of sheep which they desired, 

 produced in the course of years two flocks which differed 

 markedly from one another, so that the veriest tyro could 

 distinguish them. The origin of the principal breeds of 

 domestic animal is lost in the mist of primeval history. 

 In the case of the pigeon and of the dog they differ so much 

 from one another that it has been often believed by breeders 

 that they must have originated from distinct species ; neverthe- 

 less Darwin showed by convincing argument that this cannot 

 have been the case with the breeds of pigeon, but that they 

 must all have arisen by the mating together of selected in- 

 dividuals belonging to one species. Thus he argued that the 

 principal breeds, as for instance the pouter with its enormous 

 crop, and the fantail with its colossal tail, exhibit features which 

 are shown by no wild species of pigeon whatever. They differ 

 from the wild rock-pigeon in characters sufficiently important 

 to distinguish not merely species but genera from one another. 

 Nevertheless they interbreed freely with the rock-pigeon, 

 and have clearly been derived from that species. The ques- 

 tion then arises, whether if man by "artificial selection "" can 

 create forms so different from one another as the fantail 

 pigeon and the pouter pigeon, any process of natural selection 

 exists which in course of time could cause the offspring 

 of the same ancestor to differ as widely from each other as 

 existing species do? lhat such a process does exist is the 

 great point of Darwin's book. He lays stress on what we 

 have already pointed out in this work, viz. that every species 

 of animal tends to multiply itself so rapidly that it would 

 soon overrun the globe unless it were checked, but that never- 

 theless the numbers of any wild species at present living do 

 not vary much from year to year. This means that its num- 

 bers must be subject to constant depletion through the dangers 

 to which they are exposed. Many individuals are devoured 

 by enemies, many die from starvation during periods of 

 scarcity, or from cold or heat during extremes of temperature, 

 and the race will ultimately be propagated by those members 

 which are best fitted to overcome and survive these troubles. 



