GEOLOGY AND EVOLUTION. 373 



mere succession towards the likeness of existing life. If we turn to 

 the oldest rocks, we find the fossils to include only those of low or 

 primitive animals and plants. It is only as we pass to the newer 

 rocks that we find traces of higher life. If we select any class of 

 animals or plants, we do not discover, as would be expected if the 

 theory of " special creation " were true, that all ranks and stages in 

 that class appear simultaneously in the fossil record. On the con- 

 trary, the lower forms invariably precede the higher. Fishes, as the 

 lowest Vertebrates, thus precede other members of their type in time. 

 The oldest fossil fishes occur in the Upper Silurian rocks ; and it is 

 not until we arrive at the newer Coal measures that we find the suc- 

 ceeding and higher class, that of the frogs or amphibians represented. 

 In the deposits (Permian) above the Coal, the still higher reptiles first 

 occur ; birds make their first appearance in the Mesozoic rocks, and 

 the oldest mammals or quadrupeds are also of Mesozoic age. What 

 holds true of Vertebrate development, applies also to the appearance of 

 life at large on the earth's surface. Where the fossil evidence that 

 the lower members of a class appear before the higher seems doubt- 

 ful, the discrepancy must be referred to that imperfection of the 

 fossil record which constitutes an impassable barrier to our full and 

 complete knowledge of the life of the past. But wherever sufficient 

 materials are to be found, the great law of progression from lower to 

 higher types of life is seen to be paramount as an expression of the 

 manner in which the world has attained the fulness of its existing popu 

 lation. It need hardly be pointed out how powerfully the discovery of 

 this progression in the past life of the globe supports the evolutionist's 

 views. Instead of the sudden appearance of whole groups of high 

 and unaltering organisms in the oldest formations, we are enabled to 

 trace the gradual development of species. In the case of certain 

 groups of animals, the evidence for the theory of development be- 

 comes singularly complete when we discover the exact course in 

 which the evolution of new species by the modification of the old 

 has taken place. Amongst the extinct trilobites and ammonites, for 

 instance, the gradations between the various forms are often traceable 

 with singular completeness ; the gaps between the different species 

 being frequently supplied in the most exact, fashion. 



A notable feature in the life of the past consists in the observa- 

 tion that many extinct animals present characters which clearly 

 belong to the young or embryonic condition of their type, rather 

 than to the full-grown state, or to that seen in their existing representa- 

 tives. This fact constitutes in itself a singularly powerful argument 

 in favour of the evolutionist's views. It shows that the progression 

 of life in time past has been that which the development of animals 

 to-day demonstrates. In other words, as the stages in development 

 we see to-day repeat the ancestry of the developing animal, so these 



