HIGH SCHOOL ZOOLOGY. 269 



most or most modern layers will be most like those that we 

 actually see in process of formation, while the oldest or lowest 

 will have been subjected to many changes, such as pressure 

 from those above, upheaval from disturbances of the earth's 

 crust, etc. Now, the examination of the remains of organic 

 life — fossils — preserved in these sedimentary rocks (an import- 

 ant branch of Biology — Palaeontology — ) shows that only 

 a comparatively thin layer of superficial deposits contains 

 remains of organisms like those at present alive ; the deeper 

 down we go, the more do we find diflferent species of plants and 

 animals replacing those familiar to us, until the latter disap- 

 pear completely and leave us in the presence of an entirely new 

 fauna and flora. The question now arises, — have the species of 

 plants and animals now living on the surface of the globe, and 

 whose remains are found in the superficial deposits, originated 

 entirely independently of the different species found in deeper 

 strata, or are the ancestors of the former really represented 

 among these unfamiliar remains, only uni-ecognizable at first 

 on account of differences, which the recent forms have accumu- 

 lated in the coui-se of long ages 1 The latter solution of the 

 question is the alternative generally accepted at the present 

 day, and it will be seen that it involves relationship by descent 

 of the present species of plants and animals to some of those ot 

 former epochs, others having died out or become extinct with- 

 out leaving any descendants, just as has been the case within our 

 knowledge of modern species (V, 15, 23 ; VI, 22). Not only 

 does it involve descent, but descent with modification, and indeed, 

 when we look at the forms of life in the older rocks we recugnize 

 that the modification must have been of a very profound nature. 

 It has, furthermore, been of a definite, orderly character, 

 because we find that the lower classes of plants and animals 

 have appeared before the higher classes — -fossil Mammalia, for 

 example, being found only in comparatively recent strata. 

 There has thus been (if we accept the view that the present 



