2o8 The Study of Animal Life part in 



(p) It is said that the occurrence of Fishes in the 

 Silurian, and of many highly organised Invertebrates in the 

 still earlier Cambrian, is inconsistent with a theory which 

 would lead us to expect very simple fossil forms to begin 

 with. But to say so is to forget that we have no concep- 

 tion of the vast duration of periods like the Silurian and 

 Cambrian, while the antecedent Archasan rocks in which we 

 might look for traces of simple ancestral organisms have 

 been shattered and altered too thoroughly to reveal any 

 important secrets as to the earliest animals. 



(f) It is maintained that organic evolution proceeds very 

 slowly, and that the geologists and biologists demand more 

 millions than the experts in astronomical physics can grant 

 them. But there is considerable difference of opinion as to 

 the unthinkable length of time during which the earth may 

 have been the home of life ; we are apt to measure the rate 

 of evolutionary change by the years of a man's lifetime which 

 lasts but for a geological moment ; and there is reason to 

 believe that the simpler animals would change and take 

 great steps of progress much more rapidly than those of 

 high degree. 



6. Relative Antiquity of Animals. — I have not much 

 satisfaction in submitting the following table showing 

 the relative antiquity of the higher animals. Such a table 

 is only an approximation ; it does not suggest the great 

 differences in the duration of the various periods, nor how 

 the classes of animals waxed and waned, nor how some types 

 in these classes dropped off while others persisted. But the 

 general fact which the table shows is true, — in the course 

 of time higher and higher forms of life have come into 

 being. It is true that the remains of mammals are of more 

 ancient date than those of birds, but it is likely that the 

 remains of the earliest birds have still escaped discovery ; 

 moreover, the earliest known mammalian remains seem to 

 be of those of very simple types. 



