DISTRIBUTION OF MOLLUSCA IN TYME. 121 
aud below; the locality of the newest rock in which it occurs 
being often far removed from that of the oldest.* 
That species should be created at a single spot, and gradually 
multiply and diffuse themselves, is sufficiently intelligible. That, 
after attaining a certain climax of development, they should 
decline and disappear, is a fact involved in mystery. But even 
if it depends on physical causes, and is not a law of all Being, its 
operation is equally certain, and does not appear to vary beyond 
moderate limits. 
- The deep-sea shells (such as Rhynchonella, Terebratula, and 
Yoldia) enjoy a longer range in time, as well as in space, than 
the littoral species; whilst the land and fresh-water shells are 
most remarkable for specific longevity.t 
In each stratum there are some fossils which characterise 
small subdivisions of rock, just as there are living species of 
yery limited range. 
When species once die out they never reappear; one evidence 
of their haying become extinct consisting in their replacement 
by other species, which fulfilled their functions, and are found 
in deposits formed under similar conditions. (Forbes.) 
The total number of species is greater in the newest forma- 
tions than in those of older date; but the ratio of increase has 
not been ascertained.t 
Distribution of Genera in Time.—The doctrine of the Identi- 
fication of strata by fossils derives its chief value from the fact 
that the development and distribution of genera is as much sub- 
ject to law as the distribution of species; and, so far as we know, 
follows a similar law. 
Groups of strata, ike the zoological provinces, may be of 
yarious magnitudes; and whilst the smaller divisions are cha- 
racterised by peculiar species, the larger groups have distinc* 
sub-genera, genera, and families, according to their size and 
importance. 
William Smith himself observed that ‘‘three principal families 
of organised fossils occupy nearly three equal parts of Britain.” 
* M, Agassiz and Professor E. Forbes have represented, diagrammatically, the 
distribution of genera in time, by making the horizontal lines (such as in p. 124) swell 
out in proportion to the development of the genera. Those whose commencement, 
climax, and end are ascertained may be represented by a line of this kind —=y— 
Genera which attain their mazima in the present seas are thus expressed —<agg 
{+ Land and fresh-water shells of existing species are found with the fossil bones of 
the Mastodon and Megalonyx, in N. America. (Lyell.) 
} The number in each formation depends on the extent to which it has been investi- 
- gated, and on the opinions entertained as to the strata referable to it. Professor 
Phillips has discussed this ssbject in his work on Devonian fossils (p. 165), and in the 
“Guide to Geology.” 
G 
