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Show 356 GEOGRAPHICAL DISTRIBUTION. [CHAP. XII. islands possess their own peculiar species of aerial mammals or bats. We can see why there should be some relation between the presence of mammals, in a n1ore or less modified condition, and the depth of the sea between an island and the mainland. We can clearly see why all the inhabitants of an archipelago, though specifically distinct ·on the several islands, should be closely related to each other, and likewise be related, but less closely, to those of the nearest continent or other source whence imrnigrants were probably derived. We can see why in two areas, however distant from each other, there should be a correlation, in the presence of identical species, of varieties, of doubtful species, and of distinct but representative species. As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space: the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas. We see this in many facts. The endurance of each species and group of species is continuous in time ; for the exceptions to the rule are so few, that they may fairly be attributed to our not having as yet discovered in an intermediate deposit the forms which are therein absent, but which occur above and below : so in space, it certainly is the general rule that the area inhabited by a single species, or by a group of species, is continuous; and the exceptions, whicn are not rare, may, as I have attempted to show, be accounted for by migration at some former period under different conditions or by occasional1neans Qf transport, and by the species having become extinct in the intermediate tracts. Both in time and space, species and groups of species have th~ir points of maximum development. Groups of species, belonging either to a certain period of time, or to a certain area, are often characterised by trifling characters in common,, as of scnlptu~·e or colour. In looking to the long succession of ages, as Ill now looking to distant provinces throughout the world, we find that some organisms differ little, whilst others belonging to a different class, or to a different order, or .. 0HAP. XII.] SUMMARY. 351 even only to a different famil f greatly. In both time and s Y 0 the same order, differ each class. generally change P1~~= !~e lo'1er ~embers of there are In both cases marked ant~ e higher; but On my theory these several rel e~cep Ions to the rule. and space are intelliO'ible. for a~I~~s throughout time forms of life which h~ve chan ed e .er we look to the within the same quarter of tfe dy~Ing successive ages have changed after havin mi ra~or. 'or ~o those which in both cases the forms -#ithi~ ea~~ I~to ~stant quarters, nected by the same bond of ordinar c ass aye been conmore nearly any two forms are rel [ J'~ne~ition ; and the er they. will generally stand to e:ch 1~h 0 ?d, ~he nearspace ; In both cases the laws of va . ~ er h In time and same, and modifications have be ria lOll rve been the same vower of natural selection. en accumu ated by the |