370 SUMMAKY OF THE LAST Ciiai-. XII. 



peculiar; aii<l 'wliy, in rclalion to tlie means of iiii<>ration, one 

 group of beings, even Avithin the same class, should have all its 

 species peculiar, and another group should have all its species 

 the same Avith those in other quarters of the world. We can 

 see why "whole groups of organisms, as batrachians and terres- 

 trial mammals, should be absent from oceanic islands, Avhilc the 

 most isolated islands should possess their own peculiar species 

 of aerial mammals or bats. We can see why there should be 

 some relation between the presence, in islands, of mammals, in 

 a more or less modified condition, and the depth of the sea be- 

 tween such islands and the main-land. We can clearly see why 

 all the inhabitants of an archipelag-o, though specifically distinct 

 on the several islets, should be closely related to each other, 

 and likewise be related, but less closely, to those of the nearest 

 ccnitinent or other source whence immigrants might have been 

 derived. AVe can sec why, in two areas, however distant from 

 each other, where very closely-allied or representative species 

 exist, there should almost always exist some identical 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 successioni of fonns in past times being 

 nearly the same with those governing at the present time the 

 differences in difTerent areas. We see this in many facts, llie 

 endurance of each species and group of species is continuous in 

 time ; for the exceptions to the rule are so few, that they may 

 be fairly attributed to our not having as yet discovered in an 

 intermediate dei")Osit certain forms which are absent in it, but 

 which occur both above and below : so in space, it certainly is 

 the general rule that the area inhabited by a single species, or 

 by a grouji of species, is continuous, and the exceptions, which 

 are not rare, may, as I have attempted to show, be accounted 

 for by former migrations mider difTerent circumstances, or 

 through occasional means of transport, or by the species having 

 become extinct in the intermediate tracts. Both in time and 

 space, species and groups of species have their points of maxi- 

 mum development. Groups of species, living during the same 

 j)eri(jd of time, or living jvitliin the same area, are often char- 

 acterized by trifling features in common, as of sculpture or 

 color. In looking to the long succession of past ages, as in 

 now looking to distant provinces throughout the world, we 

 find that species in certain classes differ little from each other, 

 while others in a different class, or only in a different family of 

 the same order, differ greatly from each other. In both time 



