278 KEY TO TERRESTRIAL HISTORY 



represented in the accompanying diagram by the letter p 



EUROPE. INDIA. 



a>\ #10, PS, PS PI . put, 0,1, a t 



to indicate their association with the Pacific Ocean. In 

 the corresponding rocks of Europe certain other groups 

 of Ammonites are found which appear to be equally 

 characteristic of an Atlantic province ; they are in- 

 dicated by the letter a in the diagram. But, and this 

 is the point of real significance, in the same zone 

 as represented in the two remote localities, we find 

 an admixture of these two groups of Ammonites : in 

 Europe, though Atlantic types preponderate, yet a few 

 Pacific forms occur ; in India there is a similar col- 

 location, but with more Pacific and fewer Atlantic 

 species. 



This may be most readily explained by supposing 

 that contemporaneous migration in opposite directions 

 was proceeding while each zone was in process of 

 formation, Pacific Ammonites travelling to Europe, 

 while Atlantic Ammonites were on their way to India. 

 What, then, becomes of the argument that the presence of 

 similar species in remote regions indicates a difference 

 of age equivalent to just so much time as was required 

 for these species to migrate from one area to the other ? 

 Since the Atlantic species a lt a 4 , spent time in accom- 

 plishing the journey from Europe to India, the beds 

 in which they occur in India must, on this showing, 

 be older than those in Europe by this interval ; but again, 

 since the Pacific species p 2 , p 3 , require time to travel 

 from India to Europe, the beds in Europe must be 

 older than those in India, and thus each must be 

 older than the other, which is impossible. 



The apparent absurdity arises, however, to some 



