656 ZOOLOGY sect. 



Detailed study of the geographical distribution of species and 

 varieties in certain regions, more especially in the United States of 

 America, has afforded much support to the view that the develop- 

 ment of new forms takes place as a result of the appearance of 

 varieties differing slightly from the parent stock and their isolation 

 from the latter by geographical barriers ; and by some writers 

 evolution is even supposed to have proceeded solely, or almost 

 solely, in this way with little, or entirely without, aid from natural 

 selection. Another kind of isolation might be supposed to take 

 the place of geographical in preventing the inter-crossing of new 

 varieties with the original stock. By means of sexual or 

 jj/iysiological isolation, i.e., hy the form in question becoming varied 

 in such a way that it does not readily inter-breed with the main 

 stock, the new variety may be as effectively isolated as if separated 

 from it by a geographical barrier. 



That allied species and sub-species differ in their geographical 

 range, and are often separated by geographical barriers ; and that 

 a physiological barrier may be set up between allied forms owing 

 to union between them being impossible or sterile, are facts of 

 great importance in the study of evolution, and the details of such 

 cases are necessary data of the science. But that the whole of 

 organic nature should have been evolved by variation and isolation 

 alone seems to be highly improbable : such a view takes no account 

 of progressive adaptation and orthogenetic development, and it 

 would seem to call for the formation of an impossible succession of 

 barriers. 



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 

 or less widely divergent character in the male and female. 

 Between the males of animals of many groups, contests frequently 

 take place, and this affords us an explanation of the presence or 

 special development in many cases in that sex of various offensive 

 and defensive weapons — horns, tusks, and the like. Similarly, we 

 are able to understand the greater vigour, in the majority of cases, 

 of the male, with concomitant greater intensity of coloration, and 

 the development of various ornaments and excrescences not present 

 in the female. In many groups of Insects, and in a large propor- 

 tion of Birds, sexual differences in coloration are very marked. 

 These are, in some instances, to be traced to the necessity for 

 different protective resemblances required in the two sexes owing 

 to different habits, or to the necessity for protective colorations 

 and markings in the female and not in the male. In the case 



