SOME ASPECTS OF GEOLOGIC TIME 271 



carried out since the dawn of geology. Geologists, in deter- 

 mining the age of strata, are almost entirely dependent on the 

 biologist. But for the discovery of characteristic fossils, they 

 would, in many cases, be without the slightest clue to the age of 

 particular formations. And, by this method, it has been possible 

 to divide geologic time, not only into the broad recognised 

 epochs, but into a varying number of zones. This line of 

 investigation appears to be open to further development. 



A useful and striking example, which has recently been very 

 ably popularised by Prof. Sollas, 1 is found in the famous Oppel 

 zones of the Jurassic. No less than thirty-three distinct zones 

 have beenidentifiedbyobservingthe structure of fossil ammonites. 

 Each species is found in a particular zone, and nowhere else. It 

 has been proved that the sub-divisions are world wide. Every- 

 where, in Europe, India, America, Australia, theyfollow each other 

 in the same succession. Types like this do not arise in a day. 

 They are not distributed over the whole world in a short time. 

 Previous types are not displaced all at once. In particular 

 regions, species may be exterminated rapidly, but surely not all 

 over the world. It will be noted that these ammonites are 

 definite and distinctive types. The manner of their evolution 

 does not appear to have been determined. The minute grades 

 by means of which they must have been evolved from preceding 

 creatures have not been found. Such have probably been 

 formed locally, in some specialised and confined area, and, when 

 the barriers have been removed, the species would gradually 

 penetrate all over the world. We know little as yet of the rate 

 of the evolution of life, but the suggestiveness of these facts in 

 connection with our subject does not require to be pointed out. 

 Such facts as these have a very cogent bearing on our subject. 

 In the first place, the very existence of this continual succession 

 of organic forms is itself striking. Prof. Sollas, who is committed 

 to an unusually small estimate of geologic time, thinks that 

 these forms have succeeded each other with unusual rapidity. 

 His suggestion cannot be rejected on a priori grounds. So 

 small is our knowledge of the possible rapidity of organic 

 evolution, that we are unable to say that species may not, as he 

 surmises, have succeeded each other at intervals of 25,000 years. 1 

 The study of recent strata does not appear to have disclosed any 

 similar case of rapid evolution, but the hypothesis cannot be 



1 Age of the Earth, pp. 273 seq. 



