140 THE PLAXT WORLD. 



natural topographic and climatic areas, and that the variuus 

 groups and species are very closely correlatcMJ with the physical 

 conditions of their habitat. This raises questions which, as 

 stated hy the author/ strike at the very root of matters that arc at 

 present rather loosely discussed in current literature. 



"Can we account for their present distribution ou the hy- 

 pothesis of direct modification in response to enviromiieutal 

 stimuli accompanied by natural selection? Or must we invoke 

 the aid of unknown factors wliich Avill operate to produce species 

 * " * adapted to various remote environmental complexes, 

 and then, l)y chance dissemination, have the newly modified or- 

 ganisms, and tlie appropriate habitats brought together, with the 

 final result that the entire genus shall present a nnifonii and 

 perfect correlation and adaptation to its environment ( Are the 

 phenomena of distribution to be ex})lained on the liasis of ^-aria- 

 tion and natural selection, or on the basis of mutation and segre- 

 gation in the fittest environment?'' From ;ui enormous mass 

 of data, and especially from observed contintiity of specific difi'er- 

 entiation, the author strongly inclines to the former hy]>otliesis, 

 but frankly admits that it remains unproven, and after such an 

 extended studv of the relations of organism and habitat as we 

 are not accustomed to meet in botanical literature, it is refresh- 

 ing to read "from this we <>"et a i>eneral view, a clue here and 

 there," but ''at no point are we al)le to put our finger upon any 

 one fact and say with any certainty that this is the result of that 

 factor in the environment or of any method of evolution." Yet 

 it is safe to sav that even the monumental work of T>e Vries 

 leaves no clearer impression than does this of a definite advance 

 toward a solution of the problem of evolution. 



In the Avork just referred to, the ten criteria f<;)r the deter- 

 mination of the center of dispersal of the species of a genus, as 

 given by Adams, are examined by Tower in their applicability 

 to the genus Leptinutarsa, and, excluding those that are of doubt- 

 ful value, he fastens upon four that he considers to cover fully all 

 cases, viz. : 



(1) Location of greatest difi'erentiation of a type. 



