364 CAMBRIAN PERIOD 



systems of high development, as implied by eyes and other sense- 

 organs, devices for capturing and ingesting food, organs of digestion, 

 secretion, excretion, and respiration. The Cambrian animals had 

 acquired the various habits of life possessed by existing animals of 

 their kind, as well as the various modes of preserving their lives. 



The question may be approached in another way. The studies 

 of recent decades have convinced investigators that later forms of 

 life were derived from earlier ones by processes of evolution. The 

 exact methods of evolution are not altogether understood, but the 

 fact of evolution is not now regarded as an open question. As the 

 various forms developed and diverged from a common ancestral 

 stock, many of the intermediate forms disappeared, and. the forms 

 which persisted became widely separated. By continued diver- 

 gence, with the loss of intermediate types, a discontinuous series of 

 forms was developed, and those which lived on became more and 

 more unlike. 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 uppermost and outermost 

 twigs which represent the living phase. 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 is assumed that the whole system of living things was derived 

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

 comparison of the primitive state with the degree to which life had 

 advanced in the Cambrian period will give some impression of the 

 amount of pre-Cambrian evolution. If to this be added a compari- 

 son between the Cambrian life and that of today, an estimate of the 

 relative amount of evolution before and since the Cambrian may 

 be made. 



It is to be noted that not only were all the animal sub-king- 

 doms, save perhaps the vertebrate, present, but that, in many of 

 them, the species had come to have nearly the aspect of living 

 forms. The initiation and divergence of the structures and types that 

 preceded the Cambrian stage mean much more in the way of evolution 

 than all the evolution of later times. These considerations lead to 

 the conclusion that life must have been in existence a very long time 

 prior to the Cambrian period. 



The succession of faunas. Under the doctrine of evolution, it 



