68 
ISLAND LIFE. 
[part I. 
lias been named Heliodus by Professor Newberry. Thus an 
enormous range in time is accompanied by a very wide and 
scattered distribution of the existing species. 
Whenever, therefore, we find two or more living genera be- 
longing to the same family or order but not very closely allied 
to each other, we may be sure that they are the remnants of a 
once extensive group of genera ; and if we find them now 
isolated in remote parts of the globe, the natural inference is 
that the family of which they are fragments once had an area 
embracing the countries in which they are found. Yet this 
simple and very obvious explanation has rarely been adopted 
by naturalists, who have instead imagined changes of land and 
sea to afford a direct passage from the one fragment to the 
other. If there were no cosmopolitan or very wide-spread 
families still existing, or even if such cases were rare, there 
would be some justification for such a proceeding ; but as about 
one-fourth of the existing families of land mammalia have a 
range extending to at least three or four continents, while many 
which are now represented by disconnected genera are known 
to have occupied intervening lands or to have had an almost 
continuous distribution in tertiary times, all the presumptions 
are in favour of the former continuity of the group. We have 
also in many cases direct evidence that this former continuity 
was effected by means of existing continents, while in no single 
case has it been shown that such a continuity was impossible, 
and that it either was or must have been effected by means of 
continents now sunk beneath the ocean. 
Concluding Remarks . — When writing on the subject of dis- 
tribution it usually seems to have been forgotten that the 
theory of evolution absolutely necessitates the former existence 
of a wliole series of extinct genera filling up the gap between 
the isolated genera which in many cases now alone exist ; while 
it is almost an axiom of “ natural selection ” that such nume- 
rous forms of one type could only have been developed in a 
wide area and under varied conditions, implying a great lapse 
of time. In our succeeding chapters we shall show that the 
known and probable changes of sea and land, the known 
changes of climate, and the actual powers of dispersal of the 
