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 Avere 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 Avild 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 



