eVIDEINCE CONCI^RNING SVOIvUTlON. II3 



DISTRIBUTION AND VARIATION AND THE EVIDENCE THEY 

 AFFORD CONCERNING EVOLUTION. 



In the first chapter and in the first portion of this one has been presented 

 the data of distribution and variation, these being the two sources from which 

 we are prone to draw extensively for evidences of evolution. When taken 

 alone, as has been seen, neither is able to yield much that is certain concern- 

 ing evolution. Considered together, they may yield important information 

 concerning evolution in organisms. Distribution and variation often, nay, 

 almost always, are able to give rather exact ideas as to what the evolution in 

 a given group has been. In this section we shall examine our data first, for 

 evidence concerning the evolution of the genus Leptinotarsa, and second, for 

 the processes which brought about that evolution. 



We can most easily and rapidl}'^ arrive at some conclusion as to the evi- 

 dence which the study of distribution and variation affords concerning the 

 evolution and phylogeny of this genus by examining a specific case with care. 

 Let us first consider the evidence in the lineata group. 



The distribution of this group of 12 species of beetles has been described 

 in the first chapter and figured on plate i. Considered from the standpoint of 

 their distribution, the whole group may be separated into three sets of species, 

 each set characteristic of a certain area. 



It has already been shown how all available evidence points to the Guate- 

 mala-Chiapas plateau as the center of origin of this group of beetles. From 

 this point iindecimlineata can be traced northward to the escarpment of Mex- 

 ico and southward to Panama. Its variations are greater in the extremes of 

 its range than in its central portion, but these variations are, as has been 

 shown in this paper, fluctuating and place variations, as far as can be deter- 

 mined at present. On the north of the habitat of undecimlineata two closely 

 related forms occur, signaticollis and angustovittata, the latter being a varia- 

 tion from undecimlineata. We have seen how these three forms, in their posi- 

 tions on the slopes of the Mexican highlands, are, first, undecimlineata at the 

 lowest points; second, angustovittata midway, and third, signaticollis at the 

 lower edge of the escarpment. Among these three forms there are many 

 close resemblances; the chief differences are those in the elytral ornamen- 

 tation, the color pattern, and the punctation. These facts argue strongly 

 that these three species represent one line of descent. In this case we have 

 the rather unusual observation of one species arising directly from another, 

 which lends most convincing support to our argument. But to consider 

 only the facts from distribution and variation. The distribution of the three 

 species like windrows along the slope of the Mexican highlands, and the fact 

 that they show in their variations a series of modifications which overlap and 

 are in one direction, offer a condition not easy to explain otherwise than upon 

 the assumption that the species and their variations as we now find them rep- 



