278 GEOLOGY. 



were the intermediate or connecting forms dropped out, but many of 

 the branches themselves disappeared, and the remaining ones became 

 more and more widely distinct from one another. The process was 

 not unlike the evolution of a tree-top, in which the dying out of most 

 of the interior branches leaves a few great limbs which bear the more 

 numerous and more recent branches, while these in turn bear the upper- 

 most and . outermost twigs which represent the living phase. In this 

 way, or in some such way, it is thought that the existing divergence 

 of organisms into kingdoms, branches, classes, orders, families, genera, 

 species, and varieties came to be established. 



If it be assumed that the whole system of living things has been 

 derived from a common primitive form or a few primitive forms, a 

 comparison of the primitive state with the degree to which divergence, 

 and the loss of the intermediate forms, had gone in the Cambrian times, 

 will give some impression of the amount of evolution already accom- 

 plished. If to this be added a comparison between the Cambrian life 

 and the present life, an estimate of the relative amount of evolution 

 before and since the Cambrian period may be made. This will be 

 especially instructive, as it will give some impression of the relative 

 importance of the pre-Cambrian and the post-Cambrian portions of 

 the earth's history, measured by life development. 



To be sure, it cannot be assumed safely that the rate of evolution 

 in early and in late times was precisely the same, or that the evolution 

 of one branch proceeded at the same rate as that of other branches, 

 for there certainly were marked differences; but notwithstanding this, 

 a general and not altogether incorrect impression of the relative im- 

 portance of the unknown pre-Cambrian evolution, and of the known 

 post-Cambrian evolution may be derived, and this, notwithstanding 

 all elements of uncertainty, is of no small value. 



The scantiness of plant fossils. — On the most general inspection 

 of the record there appears at once an obvious inconsistency in that 

 the animal kingdom is fairly well represented, while plant remains 

 are barely identifiable; indeed, their presence would perhaps be doubted 

 if there were not imperative reasons for believing that they were present 

 in abundance. As all animals are dependent directly or indirectly on 

 plants for food, it must be supposed that plants were present in suffi- 

 cient numbers not only to support the animals, but to furnish a surplus, 

 since a portion inevitably escaped consumption by the animals. This 



