SYSTEM EQUIVALENT TO PERIOD 277 



to surpass the bounds of possibility. It may be urged, 

 perhaps, that the conditions here indicated are not pre- 

 cisely those Huxley had in mind, but it will, I think, 

 be found that, however we modify the statement, the 

 existence of a universal homotaxis will be found fatal 

 to the views the example was intended to illustrate. 

 The mere mechanical transport of dead shells would 

 almost certainly carry some species from one region to 

 another ; validity would thus be given to Heilprin's 

 argument, and inversions in the succession would almost 

 certainly be effected. Not the possible coexistence of 

 three systems, but of three zones, is the direful possibility 

 that geologists have now to face. Let us admit this 

 possibility, and even then we shall not be so badly 

 off. A Jurassic zone is only one-thirty-third of the 

 whole system, and our conclusions will not be sadly in 

 error if the period designated Jurassic in one country 

 should possibly differ in exact contemporaneity from that 

 so called in another by a fraction of this amount. Not 

 that this fraction is to be lightly disregarded ; if, accept- 

 ing the old analogy, we compare the duration of a 

 geological system to a single day, then an error of one- 

 thirty-third would be equivalent to one of twenty-two 

 minutes, too fast or too slow, in the geological clock, and 

 a deviation from the truth to this amount would scarcely 

 be tolerated even in a clepshydra. Is there, then, any 

 prospect of arriving at closer approximation ? Without 

 venturing on a positive affirmative, we may point out 

 reasons for thinking that there may. 



Thus in the Cretaceous rocks of India, which have 

 afforded to investigation the same zones as those which 

 have been distinguished in Europe, certain groups of 

 Ammonites are met with in such abundance as to suggest 

 that they originated in or about that province. India 

 is regarded as the metropolis of these forms; they are 



