60 



ISLAND LIFE. 



[part I. 



greater wariness can escape their new foes. We may thus easily 

 understand how a series of changes may occur at distant inter- 

 vals, each leading to the selection and preservation of a special 

 set of variations, and thus what was a single species may become 

 transformed into a group of allied species differing from each 

 other in a variety of ways, just as we find them in nature. 



Among these species, however, there will be some which will 

 have become adapted to very local or special conditions, and 

 will therefore be comparatively few in number and confined 

 to a limited area; while others, retaining the more general 

 characters of the parent form, but with some important change 

 of structure, will be better adapted to succeed in the struggle 

 for existence with other animals, will spread over a wider area, 

 and increase so as to become common species. Sometimes these 

 will acquire such a perfection of organisation by successive 

 favourable modifications that they will be able to spread greatly 

 beyond the range of the parent form. They then become what 

 are termed dominant species, maintaining themselves in vigour 

 and abundance over very wide areas, displacing other species 

 with which they come into competition, and, under still further 

 changes of conditions, becoming the parents of a new set of 

 diverging species. 



Definition and Origin of Genera. — As some of the most 

 important and interesting phenomena of distribution relate to 

 genera rather than to single species, it will be well here to 

 explain what is meant by a genus, and how genera are supposed 

 to arise. 



A genus is a group of allied species which differs from all 

 other groups in some well marked characters, usually of a 

 structural rather than a superficial nature. Species of one 

 genus usually differ from each other in size, in colour or 

 marking, in the proportions of the limbs or other organs, and 

 in the form and size of such superficial appendages as horns, 

 crests, manes, &c. ; but they generally agree in the form and 

 structure of important organs, as the teeth, the bill, the feet, 

 and the wings. When two groups of species differ from each 

 other constantly in one or more of these latter particulars they 

 are said to belong to different genera. We have already seen 



