THE EVOLUTION OF ANIMALS. 383 



been any change in a species; yet we do have some evi- 

 dence on a broader scale which is of a nature quite similar 

 to that in the last section. In the way in which animals 

 occur on the face of the earth a great many interesting 

 factors enter, and it sometimes happens that we get some 

 good indication of the long-time effects of variation. 

 Sometimes we find two forms of plant or animal flour- 

 ishing in two regions that are separated from each other 

 by some kind of a barrier which does not allow them to 

 pass back and forth and thus to mingle. These forms 

 are in general similar, and yet are constantly and recog- 

 nizedly different. Sometimes we can find that these 

 two regions (b and c) have been stocked from some third 

 region (a) , and that both varieties are apparently descend- 

 ents of the same ancestors. Indeed, we may be able to 

 find forms of all grades connecting "a" with "b" on one 

 side, and other intermediate types connecting "a" with 

 "c" on the other side. In cases like this, which are by 

 no means infrequent, it is believed that "b" and "c" 

 have migrated into their respective regions and in becom- 

 ing adapted to their new condition of life have so evolved 

 as to become different from their parent stock and from 

 each other. 



In a similar way, but on a much larger scale and in a 

 form too complex to make clear to the beginner, we find 

 evidences of change and development in the animals of 

 the great continental and other natural divisions of the 

 earth. 



382. Evidences from Geological History. In the rock 

 strata of the crust of the earth we find abundant plant 

 and animal remains in the form of fossils. In the most 

 recent strata we find remains similar to the species of the 



