No. 511] THE THEORY OF ORTHOGENESIS 



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possible explanation, for, according to de Vries, tlie 

 specific changes are sudden and fixed, the new race ap- 

 pearing fully formed, which is certainly not the ease in 

 these snakes, unless we consider the new race as having 

 been formed by small successive mutations of the same 

 kind, which would, if I understand him rightly, be con- 

 trary to de Vries's idea of mutations, for he says that 

 they "take place so far as experience goes without definite 

 direction." We have apparently in these snakes an 

 example of definite but gradual variation, in that there is 

 in each group a gradual development of forms along a 

 fixed path, without the aid of natural selection. 



A theory that accounts for the definite evolution of 

 this genus without the aid of selection is that there has 

 been in each group a gradual modification of the forms 

 under the influence of the environment. The deter- 

 minate variation of the average type, the close association 

 of the forms with different geographic regions and the 

 consequent correspondence of areas of transition in char- 

 acters with intermediate geographical conditions, would 

 seem to render this conclusion unavoidable, if, as seems 

 evident, selection is not operative here. It should be 

 noted, however, that, while the evolution of the genus 

 has been distinctly orthogenetic and associated with the 

 environment, it does not appear that the latter has, as 

 has often been supposed by adherents of this theory, a 

 specific effect. This would be difficult to determine if 

 we were dealing with one group, for example one that 

 pushed northward from the Mexican plateau, for here 

 there would be an increasing difference in climatic condi- 

 tions, associated with accumulated modifications, between 

 the range occupied by the form at the center and that of 

 the most outlying species. But when all four groups 

 are considered it is seen that the same modifications 

 appear whether the group is pushing into the tropic or 

 temperate regions, or into deserts, semi-arid plains, or 

 humid forests, and it is difficult to. conceive of environ- 

 mental conditions common to all of' the regions occupied 



