380 GEOLOGICAL BIOLOGY. 



tions of life are easier or more congenial to its particular con- 

 dition, but because it has more of the essence of evolution ia 

 it. 



The Philosophy of Evolution: a Summary. It is this view of 

 evolution which the geological history of organisms emphasizes. 

 When we look back historically to the early geological ages, and 

 not assuming that we have reached the beginning, but allow- 

 ing that there may have been as long a stretch of time before 

 the Cambrian as since, for organisms to evolve in, when we 

 compare the rate of initiation of characters of higher rank 

 with the rate of initiation of varietal or specific rank, we 

 find it to be a striking fact that relatively the initiation of 

 higher characters predominated in early times, and as time 

 went on differentiation in each line was confined to characters 

 of less and less taxonomic value ; to use the oft-cited figure of 

 a phylogenetic tree, all the main branches dichotomized near 

 the roots of the tree, and as we advance chronologically 

 toward the present the branching has been confined to 

 secondary and tertiary limbs and terminal twigs. Although 

 such a tree is used as a figure of the way in which differentia- 

 tion has arisen, it seems never to have occurred to those 

 adopting this analogy that all the branching of a tree is 

 peripheral, at the very terminal twigs. The bifurcation of 

 two contiguous twigs becomes the main crotch of the trunk 

 only by the circumferential growth of the twig into a great 

 limb ; but does any one imagine that the difference between 

 a Crustacean and a Pteropod was in any particular of less 

 taxonomic value in the Cambrian time than it is now? or has 

 the difference between two species of Silurian Rhynchonellas 

 become of any greater significance by the continuous evolu- 

 tion of the Brachiopods up to recent time? No; natural 

 selection only works at the adjustment of varietal modifications 

 in making them permanent, or in dropping them out of the 

 race; and the mere transmission of an insignificant character 

 from parent to offspring for a million generations cannot in 

 itself have the least effect in raising the economic impor- 

 tance of that character among the functions of its possessor. 

 It is this view of the case which shows natural selection to be 

 but one of the phenomena incident to evolution, and not the 



