24 EVOLUTION AND ANIMAL LIFE 



animals or plants to the separated twigs of a tree, of which 

 the trunk is more or less concealed. "We can only predicate 

 and define species at all/' says Dr. Elliott Cones, " from the mere 

 circumstance of missing links. Our species are twigs of a tree 

 separated from the parent stem. We name and arrange them 

 arbitrarily, in default of a means of reconstructing the whole 

 tree, in accordance with nature's ramifications." To continue 



FIG. 14. Heron flying. (After Marey.) 



the figure, in our studies of the origin of the twigs of the tree, 

 the existence of the trunk must not be forgotten. In the life 

 of the earth variety in unity, unity in variety are nowhere 

 separated. 



Another equally striking simile is this: A species is an 

 island, a genus, an archipelago, in a sea of death. The species 

 is clearly definable only as its ancestors and cousins have dis- 

 appeared, only in the degree that the stages in its development 

 are unrepresented in our records. The genus is a group of 

 species, an archipelago of islands, and there may be every 

 conceivable degree of width or breadth of channel which seems 

 to separate one island or group of islands from another. 



