VIII 



BRIEF ESSAYS 



I 



THE biologist's TEEE OF LIFE. 



/^NE of the most helpful and satisfactory con- 

 ^^ ceptions of modem biological science is the 

 conception of the animal life of the globe under 

 the image of a tree, — a tree which has its root and 

 trunk in the remote past, and its outermost twigs 

 and branches in our own day; and, moreover, a 

 tree which has attained its growth, which has 

 reached its maturity, and whose history in the far 

 future must be marked by a slow decline. This is 

 the Tree of Life of the evolutionist, and affords the 

 key to the natural classification of the animal king- 

 dom as taught by Darwin and others, and as op- 

 posed to the artificial or arbitrary classification of 

 Cuvier and the older naturalists. This tree first 

 emerges into view in the Silurian age, probably not 

 less than fifty million years ago, and emerges as a 

 pretty well-developed tree, that is, as having many 

 branches. Its trunk is beyond our ken, hidden in 

 still more remote ages. No fossils have been found 

 in rocks older than the Silurian. But if evolution 



