502 GEOLOGY 



Cambrian trilobites are known to have passed through a series of 

 remarkable changes after the individuals had developed far enough 

 to be fossilized, and it is inferred they passed through other stages 

 previously. There is, therefore, specific ground for believing that 

 they had a long line of ancestors. 



On the anatomical and physiological side, it is clear that nearly 

 or quite all the fundamental organs had been developed. There 

 were skeletal systems of several forms, muscular systems, nervous 

 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; in short, practically all the 

 great anatomical and physiological systems now possessed by 

 animals. 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. 



(2) The studies of recent decades have convinced investigators 

 that later forms of life were derived from earlier ones by processes 

 of evolution, the exact method of which is not altogether under- 

 stood; but the fact of derivation 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 disap- 

 peared, and thus the diverging forms became somewhat widely 

 separated. By continued divergence, with the loss of intermediate 

 types, a more and more discontinuous series of forms was de- 

 veloped, and those branches which lived on became more and more 

 distinct. 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 has been 

 derived from a common primitive form or from a few primitive 

 forms, a comparison of the primitive state with the degree to which 

 divergence had gone in the Cambrian times will give some im- 



