64 ZOOLOGY 



he can neither initiate or arrest, yet he is still left with a 

 large number which by no known process can be changed into 

 anything else. The remains of animals preserved in the rocks 

 teach us, however, that many species which formerly existed an 

 the earth have become extinct ; and there is a general belief 

 among geologists that none of the species now existing on the 

 earth were living when the materials making up the first 

 fossiliferous rocks were laid down as sand, mud, and gravel at 

 the bottom of seas, lakes, and rivers. This belief is based on 

 the experience that if we examine successively older and older 

 rocks, the remains of existing species become scarcer and scarcer 

 and finally disappear, whilst those of extinct species increase in 

 number. This line of argument is, however, not conclusive, 

 because although the presence of the remains of a species in a 

 certain rock proves that the species lived at the time the rock 

 was being formed, yet its absence does not prove that it was 

 not then living. Only a small proportion of the species living 

 at any one time were actually preserved as fossils ; and of the 

 fossils which actually exist, probably only a small proportion 

 have yet been discovered. Every few years fossils of some 

 type of animal are discovered at an horizon far lower than the 

 lowest at which they had been previously supposed to exist. 



There are, however, independent grounds for believing that 

 the species of animals which we now find living have not been 

 in existence since the beginning of life on the globe, but have 

 arisen gradually as time went on through the operation of 

 natural processes. These grounds are fully set forth in the 

 immortal work entitled The Origin of Species, by Charles 

 Darwin, a work which everyone interested in biology should 

 read and re-read, and to which it is hoped that the present 

 book may serve as an introduction. Darwin's work ought to 

 be regarded as the Sacred Book of every student of zoology, 

 for no work has done more to revolutionise the whole out- 

 look of man on the universe than this one. 



A very brief resume* of the principal lines of argument 

 employed by Darwin may be given here. We find, Darwin 

 says, that if a species be closely examined it is found to be 

 divisible into different " races." These races are sections of the 

 species consisting of slightly different types of individual con- 

 fined to distinct areas within the single area to which the whole 



