Il8 VARIATION IN LEPTINOTARSA. 



which are difficult to explain on any basis ; but evidence does not appear any- 

 where in the data of variation and distribution in the whole genus in support 

 of the theory of evolution by slow variations and natural selection. Are these 

 series of species, with their differentiations, in direct and orderly succession 

 from their center of origin over the country, and the continuity of their varia- 

 tions, to be taken as certain evidence in support of this view ? In any one of 

 the many examples of this kind which could be mentioned is there any more 

 probability of their having arisen by slow variations and natural selection 

 than by orthogenetic mutation ? 



In regard to the other h3^potheses of the method of evolution, do they 

 secure any support from the data under consideration? In the entire genus 

 there is found present to a marked degree evolution and fluctuating variation 

 in narrow limits in definite directions. Does it follow that orthogenesis (in 

 Eimer's sense) has been the method of evolution pursued by this genus of 

 beetles ? Excepting for the existence of definite directions of evolution there 

 is no evidence in support of this view. The determinate character of the 

 variations and species differentiation can be just as well explained on the basis 

 of slow variation and natural selection by the elimination of all variations and 

 species but those within narrow limits as upon Eimer's theory of orthogenesis. 

 It would be equally easy for the neo-Lamarckians to devise a plausible argu- 

 ment as to the origin of the genus by the method of evolution demanded by 

 their hypothesis, and likewise for the advocates of species formation by muta- 

 tions and segregation in fittest environment to make out of the data a complete 

 and satisfactory case in favor of their theory. We do know, however, that 

 certain extreme variations or mutants occur, as, for example, melanothorax 

 and ruhicunda, and while these may argue in favor of species origin by 

 mutation, in the remainder of the species no more such examples are known ; 

 and, as far as any evidence yet discovered is concerned, these have not pro- 

 duced permanent species. Moreover, the origin of the species in the genus 

 can be explained by any one of the hypotheses we may choose, or ruhicunda 

 and melanothorax may be passed by as exceptional cases and of little sig- 

 nificance. 



Viewing the data and evidence from a strictly impartial standpoint, as one 

 having no more interest in one hypothesis than in any other, as far as I can 

 discern there is no certain basis for asserting what method or methods of evo- 

 lution have been followed in this genus of beetles. Some species, multitce- 

 niata and decemlineata, afford evidence strongly opposed to evolution by slow 

 variation, while others, melanothorax and rubicunda, support the theory of 

 evolution by mutations ; but, although this evidence is suggestive, it is well 

 to keep in mind the fact that there are other possible methods of evolu- 

 tion. Although frequently far-reaching conclusions are based upon evidence 

 derived from these sources, such a proceeding is not justified. The neo- 

 Lamarckian is prone to draw from variations distributed in time and space 



