DISTRIBUTION OF MOLLUSCA IN TIME. 419 



It is now generally admitted that the earlier forms of life, strange as 

 many of them seem to us, were really less metamorphosed — or departed less 

 widely from their ideal archetypes — than those of later periods and of the 

 present day.* Tiie 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 specialized. {Owen) 



Migration of Species and diffusion of Genera in Former I'/w^.y.— Having 

 adopted the doctrine of the continuity of specific and generic areas, it remains 

 to be shewn that such groups as are now widely scattered can have 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.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) been thirty 

 times submerged, and has formed part of the sea-bed during two-thirds of 

 all the past geological time ; — there will be no difficulty in accounting for the 

 migration of sea-shells, or the diffusion of marine genera. 



On the other hand it may be inferred that every part of the present sea 

 has been dry laud many different times ; — on an average not less than thirty 

 times,— amounting to one-third of the whole interval since the Cambrian 

 epoch. 



The average duration of the marine species has been assumed at only one- 

 third the length of a geological period, and this harmonises with the fact that 

 so few (either living or extinct) have a world-wide distribution. 



The life of the land-snails aud of the fresh-water shells has been of longer 



* Mr Darwin has pointed out that the sessile Cirripedes, which are more highly 

 metamorphosed than the Lepadidtv, were the last to appear. The fossil mammalia 

 atford, however, the most remarkable examples of this law. At the present day such 

 an animal as the three-toed horse (Hippoiheriuni) of the Miocene Tertiary would be 

 deemed a lusus jialurai, but in truth 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 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 primaeval seas. 



