Chemical Basis of Genus and Species 43 



(estimated on the rate of formation of radium from 

 uranium) may be about two hundred million years and 

 estimated on the basis of sedimentation sixty million 

 years. And yet these invertebrates are so closely 

 related to the forms existing today that the systematists 

 have no difficulty in finding the genus among the modern 

 forms into which each of these organisms belongs. W. 

 M. Wheeler, in his investigations of the ants enclosed 

 in amber, was able to identify some of them with forms 

 living today, though the ants observed in the amber 

 must have been two million years old. The constancy 

 of species, i. e., the permanence of specificity may there- 

 fore be considered as established as far back as two or 

 possibly two hundred millions of years. The definite- 

 ness and constancy of each species must be deter- 

 mined by something equally definite and constant in 

 the egg, since in the latter the species is already fixed 

 irrevocably. 



We shall show first that species if sufficiently sepa- 

 rated are generally incompatible with each other and 

 that any attempt at fusing or mixing them by grafting 

 or cross-fertilizing is futile. In the second part of the 

 chapter we shall take up the facts which seem destined 

 to give a direct answer to the question as to the cause 

 of specificity. It is needless to say that this latter 

 question is of paramount importance for the problem 

 of evolution, as well as for that of the constitution of 

 living matter. 



