DISTRIBUTION OF THE MOLLtJSCA IN TIME. 129 



and liabit 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 fijfst developed are most like the embryonic forms of their 

 respective groups, and the progression observed is from these 

 general tyj)es to forms more highly specialised. (Owen.) 



Migration of Species and diffusion of Genera in Former Times. — 

 Having 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 have been difi'used 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.-f 



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 

 elevated 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 Lepadidce, were the last to appear. The fossil mammalia 

 afford, however, the most remarkable examples of tliis law. At the present day such 

 an animal as the three-toed horse {Hif-pothernmi) of the Miocene Tertiary would be 

 deemed a lusus natura, but in tnith the ordinary horse is far more 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 sei;eral distijict 

 types, and combined the character of several classes. 



t The enormous thickness of the older rocks in all parts of the world has been held 

 |o indicate the prevalence of deep water in the primecval seas. 



G 3 



