410 OEOGEAPHICAL DISTEIBUTIO^L Chap. XII. 
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 inha¬ 
bited by a single species^ or by a group of species, is 
continuous; and the exceptions, which are not rare, 
may, as I have attempted to show, be accounted for by 
migration at some former period under different con¬ 
ditions or by occasional means of transport, and by the 
species having become extinct in the intermediate tracts. 
Both in time and space, species and groups of species 
have their 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 cha¬ 
racters in common, as of sculpture or colour. In look¬ 
ing to the long" succession of ages, as in 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 even only to a 
different family of the same order, differ greatly. In 
both time and space the lower members of each class 
generally change less than the higher; but there are 
in both cases marked exceptions to the rule. On my 
theory these several relations throughout time and space 
are intelligible; for whether we look to the forms of life 
which have changed during successive ages within the 
same quarter of the world, or to those which have 
changed after having migrated into distant quarters, in 
both cases the forms within each class have been con¬ 
nected by the same bond of ordinary generation; and 
the more nearly any two forms are related in blood, the 
nearer they will generally stand to each other in time 
and space; in both cases the laws of variation have been 
the same, and modifications have been accumulated by 
the same power of natural selection. 
