70 The Mechanism of Evolution in Leptinotarsa 



The inertia of taxonomy and orthodox biology seems to be yielding to the 

 opinion that species in nature are not all due to dichotomy and extermination, 

 but that intermingling or crossing is a potent factor in species formation, and 

 increasingly frequent are the expressions of opinion that species may after all 

 be hybrids (Morgan) and that frequent intercrossing has occurred (Cook). 



In the lineata group of the genus Leptinotarsa, fossil evidence is lacking, and 

 I have no confidence in ontogentic sequence of characters as a basis for deter- 

 mining relationship. 



The data of distribution also are subject to valid criticism as a basis for 

 phylogeny, less valid than ontogeny ; but too much reliance can not be placed 

 upon distribution data. Nor can habitat and adaptability be of any consider- 

 able value, because the physiological capacities which make for habitat selection 

 and ecological restrictions are as fully capable, and are transferred by meta- 

 theses as often as are purely morphological characters. 



The lineata group is limited to North America ; beyond doubt the undecvm- 

 lineata division is strictly tropical grassland in distribution and ecological 

 relations. The multitceniata division, on the other hand, is always a temperate 

 grassland form, either on the high plateaus of the tropics which have a temperate 

 climate or on the plains farther north ; at any rate, it in the main is absent from 

 the areas occupied by the first division. The juncta division, in its range, gives 

 one the impression of being decadent and limited to areas of peculiar complex 

 like the coastal plains and subtropical deserts — areas of high temperature and 

 considerable humidity for a short growing season, then a long period of drought 

 and quiescence. These are observed conditions in nature where the divisions 

 present more than distributional and ecological distinctions, each having 

 marked morphological characters, and between them intercrossing is not only 

 not common, but is difficult in most instances to obtain. 



In each of the three divisions of the group there is an undefinable impression 

 of a fundamental basis, a form nucleus in each upon which the array of shifting 

 characteristics is placed ; whether this is a reality or only a product of the com- 

 bined characteristics present opinions would differ, and all would be merely 

 opinion, not demonstration. Within each division, however, the arrangement 

 of the characteristics gives species, which cross and show metatheses of their 

 attributes in the products of crossing. 



I could, of course, create a phylogeny for these materials, but it would be 

 necessary to assume certain premises as true which could not be tested, and too 

 much of this sort of biology has already been created. In their earlier history 

 this general group, the Chrysomelinae, reaches far back towards the base of the 

 Mesozoic era, and there the genera show species distinguished by much the 

 same characteristics as distinguish species and genera to-day. Since the rise 

 of the Chrysomelinas no one has any conception of the myriad of species and 

 lesser divisions that have come and gone in their history, and yet there persists 

 through all a basis of form, and the array of attributes and qualities capable of 

 metathetic shifting and recombination, giving species and varieties. The time 

 has passed when schemes of phylogeny can be of interest and current assump- 

 tions of monophylitic origin and dichotomy, either slow or rapid, are vanishing 

 into meaningless metaphysics. 



That there are long-continued lines of descent no one doubts ; and perhaps 

 all of our trends of evolution and also phyla are but syntheses of factors from 



