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 w T ell 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 
