652 ZOOLOGY sect. 



are still more striking. The domestic Dog, for example, exhibits a 

 large number of very marked varieties. Though all these seem to 

 be fertile "with one another, and to produce fertile offspring, it is 

 generally supposed that they have been derived from several wild 

 species with more or less hybridisation. But the enormous 

 differences which are to be observed between some of the varieties 

 have been produced to a great extent under domestication. These 

 are not all mere superficial differences, but involve also the 

 proportions and shape of the parts of the skeleton. The difference 

 in the form of the skull and in the proportions of the bones of the 

 limbs between a Greyhound and a Bulldog, for example, are very 

 remarkable — so great, in fact, that if they were found to occur 

 between two wild forms they would justify a zoologist in referring 

 the two to distinct genera. Sheep and Cattle, Pigs and Horses, 

 present similar, though not perhaps quite so strongly-marked, 

 varieties. One of the most remarkable cases of variation under 

 domestication, and one to which Darwin paid a good deal of 

 attention, is that of the domestic Pigeon. Of this, there are a 

 considerable number of varieties, known to fanciers as pouters, 

 fantails, carriers, tumblers, and so on ; and it appears to be almost 

 certain that these are descended from one wild species — the blue 

 Rock-pigeon. 



These varieties, and many more that might be mentioned, have 

 been produced by man selecting those forms that tended to vary 

 in a desired direction — have been produced, that is to say, by 

 artificial selection, sometimes consciously exercised, sometimes, no 

 doubt, unconsciously. This process has had a long period of time 

 for its operation, many of our domestic animals and plants having 

 been the objects of care and cultivation in Egypt and Western 

 Asia certainly several thousand years ago ; in many cases the wild 

 forms from which they were developed appear to have become 

 totally extinct. 



But variation occurs among animals and plants not only under 

 domestication ; it occurs also in a state of nature. Evidence of 

 this has already been adduced in the account of certain of the 

 examples of the various phyla ; and in the examination of specimens 

 of these in the laboratory the student can hardly have failed to 

 notice the occurrence of individual differences not due to differences 

 in sex or age in animals of all classes. In this respect, in the 

 strength of the tendency to individual variation, there is a very 

 great inequality between different species of animals, some being 

 extremely variable, some comparatively stable. Variations of 

 external parts have naturally, from the greater ease with which 

 they may be observed, attracted most attention, but the ex- 

 amination of the internal parts in large numbers of individuals of 

 the same species, when it has been carried out, has shown that 

 variations in internal organs are also of great frequency. 



