396 
DARWINISM 
CHAP. 
about twenty genera. Nevertheless, a great gap still exists 
between these mammals and those of the Tertiary strata, since 
no mammal of any kind has been found in any part of the 
Cretaceous formation, although in several of its subdivisions 
abundance of land plants, freshwater shells, and air-breathing 
reptiles have been discovered. So with fishes. In the last 
century none had been obtained lower than the Carboniferous 
formation ; thirty years later they were found to be very 
abundant in the Devonian rocks, and later still they were 
discovered in the Upper Ludlow and Lower Ludlow beds of 
the Silurian formation. 
We thus see that such sudden appearances are deceptive, 
and are, in fact, only what we ought to expect from the known 
imperfection of the geological record. The conditions favour¬ 
able to the fossilisation of any group of animals occur com¬ 
paratively rarely, and only in very limited areas; while the 
conditions essential for their permanent preservation in the 
rocks, amid all the destruction caused by denudation or meta¬ 
morphism, are still more exceptional. And when they are 
thus preserved to our day, the particular part of the rocks in 
which they lie hidden may not be on the surface but buried 
down deep under other strata, and may thus, except in the 
case of mineral-bearing deposits, be altogether out of our 
reach. Then, again, how large a proportion of the earth 
consists of wild and uncivilised regions in which no exploration 
of the rocks has been yet made, so that whether we shall find 
the fossilised remains of any particular group of animals 
which lived during a limited period of the earth's history, and 
in a limited area, depends upon at least a fivefold combination 
of chances. Now, if we take each of these chances separately 
as only ten to one against us (and some are certainly more 
than this), then the actual chance against our finding the 
fossil remains, say of any one order of mammalia, or of land 
plants, at any particular geological horizon, will be about a 
hundred thousand to one. 
It may be said, if the chances are so great, how is it that 
we find such immense numbers of fossil species exceeding in 
number, in some groups, all those that are now living? But 
this is exactly what we should expect, because the number of 
species of organisms that have ever lived upon the earth, since 
