616 ZOOLOGY SECT. 



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. 



Among the Protozoa, the Foraminifera are characterised by 

 numerous and marked variations so marked as " to include, not 

 merely those differential characters which have usually been 

 accounted specific, but also those upon which the greater part of 

 the genera of this group have been founded, and even, in some 

 instances, those of its orders!' The Mollusca vary also very 

 frequently and extensively, especially in the form and markings of 

 the shell; and of some of the species which have been most 

 completely studied in this respect a very large number of more or 

 less strongly marked varieties have been recorded. Many of the 

 Crustacea are also extremely variable in coloration and in the 

 length and proportions of the various appendages. But, among 

 the Arthropoda, it is in the Insecta, and more especially the 

 Lepidoptera, that we find the most striking instances of variation. 

 In the Vertebrata, also, variations in colour and proportions, as well 

 as in internal organs, occur frequently in all classes. 



