EVIDENCE CONCERNING EVOLUTION. WJ 



along certain definite lines and in correlation with the natural features of its 

 habitat. The question is whether the many series of species found represent 

 chance arrangements, or whether they indicate species differentiation along 

 definite lines, in correlation with the climatic and topographic areas of their 

 habitat. If in the genus there were only one or two such cases, we might 

 perhaps conclude that they were the result of chance; but when the entire 

 genus shows this phenomenon it is highly improbable that the conditions 

 could be due to chance modifications. On the other hand, it is almost certain 

 that the interpretation of the relationships and the direction of the evolution 

 of species indicated in text-figure 6 is, in the main, correct. In this genus, at 

 least, the study of distribution and variation gives clear ideas as to what the 

 course of evolution has been. We are fortunate in that there are few gaps 

 due to apparent extinctions and in the unusually large amount of exact and 

 reliable data available. 



Even though the truth of the above conclusion is admitted to be incontest- 

 able, no evidence is produced as to the methods by which the evolution of the 

 genus has been brought about. As far as the above conclusion regarding 

 phylogeny is concerned, it could be explained by any one of the current 

 hypotheses concerning evolution, at least as far as any facts yet discovered 

 are concerned. Slow, accumulated variations and natural selection, directive 

 evolution (Eimer's orthogenesis), neo-Lamarckism, and the mutation theory 

 would each be able in the opinion of its supporters to explain the condi- 

 tions found. 



Can we, in the study of evolution as found in this genus, derive from the 

 data of distribution and variation any evidence as to the method of evolution? 

 This is often attempted, but the conclusions are badly strained and distorted, 

 and not infrequently are entirely unfounded. Nowhere in the data presented 

 is there anything to indicate that slow variations and natural selection has 

 been the method of evolution. We have already considered this case in part 

 in the discussion of place and geographical variations, where it has been shown 

 that as far as can be determined slow variations in a given direction have 

 not been incorporated into the species, even after many hundreds of genera- 

 tions. The evidence derived from the place and geographical variations of 

 L. multitaiiiata and dccemlineata would be difficult to explain away by this 

 theory. Its advocates can, and probably will, take refuge in the argument, 

 frequently employed, that we have not evidence covering a sufficiently long 

 period of time ; but they have not yet defined what is meant by "a sufficiently 

 long period of time." We have in dccemlineata one of the most satisfactory 

 cases known of those which are calculated to give evidence on this point, with 

 complete data and actual specimens for many generations, together with mod- 

 ifications of the required kind. It is shown conclusively, howevei, that the 

 changes have not as yet become a part of the species. It may be suggested 

 that perchance we may be dealing with one of those rare and special cases 



