222 GENERAL BIOLOGY 



II. GENERAL EVOLUTIONARY PHENOMENA AS ILLUSTRATED 

 IN BRIEFER SERIES OF ORGANISMS. 



In the foregoing studies we have given brief consideration 

 to a very few plants and animals, selected to illustrate the 

 two main lines of organic development, corresponding to 

 the plant and animal "kingdoms"; but the wide gaps 

 between the types studied haA^e left far too much to be 

 bridged in imagination. Hydra and earthworm, or liver- 

 wort and fern, stand so far apart in point of structure that it 

 is difficult to conceive of all the forms intervening. Let us 

 now compare together some forms that are more alike in 

 order to see, if possible, the nature of the relations organisms 

 bear to each other. In so doing our attention will be given 

 to typical organic phenomena, rather than to typical organ- 

 isms. These will be grouped for convenience under three 

 heads: 



1. Divergence and convergence of development. 



2. Progressive and regressive development. 



3 . The correspondence between ontogeny and phylogeny. 



I. Divergence and convergence of development. 



Whatever our views of relationship, the series in which 

 we arrange organisms are based on the likenesses and differ- 

 ences we find to exist among them. This is classification. 

 We associate organisms together under group names 

 because, being so numerous and so diverse, it is only thus 

 that our minds can deal with them. Classification furnishes 

 the handles by which we move all our intellectual luggage. 



We base our groupings on what we know of the organisms. 

 Our system of classification is, therefore, liable to change 

 with every advance of knowledge. The earliest groupings 

 of animals were very simple and obvious; "creeping 

 things," "flying things," "fishes of the waters," etc. How 

 recently, indeed, have bats ceased to be grouped with the 



