chap, xin The Past History of Animals 207 



It is difficult to explain why some of the old types 

 disappeared. The extinction was never sudden. Formid- 

 able competitors may have helped to weed out some ; for 

 cuttlefish would tend to exterminate trilobites, and voracious 

 fishes would decimate cuttlefish, ■ just as man himself is 

 rapidly and inexcusably annihilating many kinds of beasts 

 and birds. But, apart from the struggle with competitors, 

 it is likely that some types were insufficiently plastic to save 

 themselves from changes of environment, and it seems likely 

 that others were victims to their own constitutions, becoming 

 too large, or too sluggish, or too calcareous ; or, on the 

 other hand, too feverishly active. The " scouts " of evolution 

 would be apt to become martyrs to progress ; the " laggards " 

 in the race would tend to become pillars of salt ; the 

 path of success was oftenest a via media of compromise. 

 Samuel Butler has some evidence for saying that " the race 

 is not in the long run to the phenomenally swift, nor the 

 battle to the phenomenally strong ; but to the good average 

 all-round organism that is alike shy of radical crotchets and 

 old world obstructiveness." 



5. Various Difficulties, — Nowadays it seems natural 

 to us to regard the fossils in the rocks as vestiges of a 

 gradual progress or evolution. As some still find difficulty 

 in accepting this interpretation, I shall- refer to three 

 difficulties occasionally raised. 



(a) It is said that the number of fossils in successive 

 strata does not increase steadily as we ascend to modern 

 times — that the numerical strength of the fauna is strangely 

 irregular. Thus (in 1872) it was computed that 10,000 

 species were known from the early Silurian rocks, while the 

 much later Permian yielded only 300. But those who use 

 such arguments should mention that a large number of the 

 Silurian species were discovered by the marvellous industry 

 of one man in a favourable locality, and that the rocks of 

 the Permian system are ill adapted for the preservation of 

 fossils. Moreover, we cannot compute the relative dura- 

 tion of the different periods, we cannot infer evolutionary 

 progress from the number of species, and we must make 

 many allowances for the imperfections of the record. 



