390 



GENERAL PRINCIPLES 



the others. It must be kept in mind that bones may fuse, may 

 dwindle by degeneration to the point of disappearance, and even 

 new bones may develop, which have no counterpart in other 

 animals. But such modifications do not detract from the value 

 of homologies. In the same way the skulls of these five classes 

 are also found to belong to the same type, and one can homolo- 

 gize the bones of a frog's skull with those of a dog. The resem- 

 blances between Birds and Reptiles are especially numerous in 

 parts of the skeleton, but a detailed knowledge of comparative 

 osteology is necessary to a full appreciation of such points. 



793. The plant kingdom, as a whole, does not form as perfect 

 or continuous a series as the animal kingdom, but all the 

 important breaks in the series fall below the Archegoniates. 

 The simple structure of the Algae and Fungi offers compara- 

 tively few features for comparison, and hence makes it difficult 

 to discern relationship. On the other hand, the series from 

 the liverworts to the highest flowering plants is more perfect 

 and more extensive than anything to be found among animals. 

 When we trace the gradual development of the sporophyte and 

 the corresponding reduction of the gametophyte, through the 

 various classes of Bryophyta, Pteridophyta and Spermatophyta 

 we are able to form an almost ideally perfect series. When one 

 studies this series, not only as a whole but in its details, the con- 

 viction that it represents a ''blood relationship" is irresistible. 



794. The anatomist often discovers organs which are clearly 

 without function. It is impossible to account for such organs 

 except on the ground that they are rudiments of organs which 

 were at one time functional. That such is the case is usually 

 evident by comparison with other species in which homologous 

 organs are to be found. When an organ becomes useless for 

 any reason it still persists in a more or less imperfect condition 

 as a vestige of its former state. Some examples of vestigeal 

 organs are of particular interest in the present connection. 



795. The whale is a mammal, since it suckles its young, as 



