ZONAL FOSSILS 279 



extent from the method by which it is presented. To 

 state it more exactly, we may represent the time taken 

 by the Atlantic species to reach India by x, and that 

 taken by the Pacific species to reach Europe by y, 

 then xy = d, the true difference in age of the homo- 

 taxial deposits of the two regions, probably in any 

 case a small quantity, while if x=y, d vanishes and 

 the deposits are precisely contemporaneous. 



In apposition to this conclusion we may place the 

 following question : " Were the British Cretaceous rocks 

 deposited at the same time as those of India, or are they 

 a million years younger or a million years older ? " This 

 was the question put to geologists by Huxley in his 

 famous address, and was followed by the statement that 

 they were in no way able to answer it. 



Of the many-sided results which have followed from 

 a study of the distribution of fossils, one in particular 

 seems to call for consideration. The notion that under- 

 lies the use of zonal fossils is that they were short-lived 

 species, which speedily attained a very wide distribution 

 over the globe ; but this idea is in flagrant contradiction 

 to a supposed biological law, for it is an accepted 

 maxim among biologists that a species widely distri- 

 buted in space is also widely distributed in time. It 

 would perhaps argue excessive hardihood to suggest that 

 this generalisation may rest on too slender a basis, 

 but without now entering into this question, we may 

 endeavour to find some other explanation of the un- 

 doubted fact that species which have evidently enjoyed 

 only a very brief existence have yet become very generally 

 distributed over the floor of the oceans. It may at once 

 be pointed out that it is not every sort of fossil that 

 can be made use of for the identification of zones ; 

 the two groups of organisms that have hitherto best 

 served the purpose are Ammonites and Graptolites. 



