264 VERTEBRATES. 



Evolution. In animal structure the student cannot 

 fail to discover increasing complexity. That there is 

 not one single complete and uniform series from the low- 

 est to the highest, is true ; but that there is more or less 

 regular gradation, is unmistakable. In the embryonic 

 life of one of the higher animals there is corresponding 

 gradation. In the development of the original single cell 

 (the oosperm) there is increasing complexity of structure, 

 and the main types of structure found in other animals 

 lower in the series are one by one roughly repeated. 

 Geology presents a corresponding gradation in the forms 

 of life which have appeared at different periods of the 

 earth's history, and plainly shows that the present fauna 

 of the earth is not like that of any preceding geologic 

 period. The fossil remains of animals preserved in the 

 rocks show in a general way that the simpler forms ap- 

 peared first, and that the most complex are of most recent 

 introduction. These facts, and others, seem to indicate 

 that there has been a continual progressive development 

 of animals from simple, primitive forms ; and such devel^ 

 opment is denoted by the term evolution. A partial ex- 

 planation of the manner in which evolution may have 

 come about is found in the law of natural selection, 

 which assumes that indefinite variations in animals are 

 continually occurring, and shows that such variations in 

 structure as fit an animal better for maintaining its place 

 in nature will be preserved through successive genera- 

 tions, and may become permanent. 1 



Animals and Plants. In the foregoing pages plant 

 life has been but incidentally referred to, yet enough has 

 been said to suggest the close interdependence of plants 



1 For a concise and able exposition of this law, the student is recom- 

 mended to read the chapter on "Natural Selection" in Morgan's Animal 

 Life and Intelligence. 



