246 PROGRESS OF SCIENCE IN THE CENTURY. 



on the crabs of Plymouth Harbour are beginning to 

 remedy this discreditable ignorance. Until we have 

 much information of this sort it is quite idle for one 

 biologist to say that he thinks one hundred millions of 

 years enough for the evolution of living creatures, 

 and for another to declare himself contented with a 

 grant of a quarter of that amount. 



We are certain that the evolution of backboned ani- 

 mals, from Silurian Fishes to Man, has occupied " a 

 period represented by a thickness of 34 miles of sedi- 

 ment " ; and although we are familiar with long-lived 

 types, like the tongue-shell, Lingula, which has per- 

 sisted with " next to no perceptible change " from the 

 Cambrian till to-day, we are also aware of races, like 

 some of the extinct Reptiles, which have appeared, 

 grown great, and disappeared within a relatively 

 short time, as time goes. " To select Lingula, or 

 other species equally sluggish, as the sole measure of 

 the rate of biologic change would seem as strange a 

 proceeding as to confound the swiftness of a river 

 with the stagnation of the pools that lie beside its 

 banks" (Sollas). 



The biological argument has been particularly dis- 

 cussed by Professor Poulton,* with the general result 

 that he feels it necessary to demand much more than 

 even the geologist demands. The general fact of im- 

 portance is that in the oldest fossil-containing rocks 

 we find highly specialised animals which must have 

 had a long history behind them; that in the Cam- 

 brian, Ordovician, and Silurian almost all the great 

 phyla or stocks of animals are already represented, 

 and in many cases by forms which are anything but 

 primitive. To the geologist's computation of the 

 period required to account for the strata between the 



* Address Section D, Rep. Brit. Ass., 1896, pp. 808-828. 



