DISTRIBUTION OF THE MOLLUSCA IN TIME. 129 
and habit rarely live together, but occupy distinct areas, and 
are termed “‘ representative species.” The same thing has been 
observed in the distribution of fossils; the species of successive 
strata are mostly representative. 
At wider intervals of time and space, the representation is 
only generic, and the relative proportions of the larger groups 
are also changed. 
The succession of forms is often so regular as to mislead a 
superficial observer; whilst it affords, if properly investigated, 
a valuable clue to the affinities of problematic fossils. 
It is now generally admitted that the earlier forms of life, 
strange as many of them seem to us, were really less meta- 
morphosed—or departed less widely from their ideal archetypes 
—than those of later periods and of the present day.* The 
types first developed are most like the embryonic forms of their 
respective groups, and the progression observed is from these 
general types to forms more highly specialised. (Owen.) 
Migration of Species and diffusion of Genera in Former Times.— 
Haying adopted the doctrine of the continuity of specific and 
generic areas, it remains to be shown that such groups as are 
now widely scattered can haye been diffused from common 
centres, and that the barriers which now divide them have not 
always existed. 
In the first place it .will be noticed that the mass of the 
stratified rocks are of marine origin, a circumstance not to be 
wondered at, since the area of the sea is twice as great as the 
land, and probably has always been so; for the average depth 
of the sea is much greater than the general elevation of the 
land.+ 
The mineral changes in the strata may sometimes be accounted 
for by changes in the depth of the sea, or an altered direction 
of the currents. But in many instances the sea-bed has been 
eleyated so as to become dry land, in the interval between the 
formation of two distinct marine strata; and these alterations 
are believed to occur (at least) once in each formation. 
If every part of what is now dry land has (on the average) 
* Mr. Darwin has pointed out that the sessile Cirripedes, which are more highly 
metamorphosed than the Lepadide, were the last to appear. The fossil mammalia 
afford, however, the most remarkable examples of this law. At the present day such 
an animal as the three-toed horse (Hippotherium) of the Miocene Tertiary would be 
deemed a dusus nature, but in truth the ordinary horse is farmore wonderful. Un- 
fortunately, a new “ vulgar error’ has arisen from the terms in which extinct animals 
have sometimes been described, as if they had been constructed upon several distinct 
types, and combined the character of several classes. 
+ The enormous thickness of the older rocks in all parts of the world has been held 
to indicate the prevalence of deep water in the primeval seas, 
GO 
