Materials, Their Taxonomy and Natural History 25 



subjected to tests to determine their value for experimental work. Their 

 adaptability to cultural conditions and the rapidity of reproduction were of 

 course vital points to be determined, but the gametic composition of the species 

 was of far more importance. Adaptability to cultural conditions, to a very 

 considerable extent, can be attained with most of the organisms that I have 

 attempted to use, and is thus far, in all instances in my experience, entirely a 

 matter of external conditions. 



The question of composition, however, is much more serious, and some other- 

 wise admirable materials have had to be discarded on account of the practical 

 difficulties encountered in the reduction of the material to a gametically homo- 

 geneous state. The chief difficulty has been the existence in the materials of 

 diverse minor strains which could be isolated by one process or another and then 

 maintained as " pure cultures." Species from nature that had no habitudinal 

 races or modifications of any kind, that were of low somatic variability in 

 response to the changing conditions of the medium, nutrition, and kindred 

 environmental factors, and free from minor strains, biotypes, or other sub- 

 divisions, have been the type of materials which I have sought for use. 



With materials of this character, and with a knowledge of the total range of 

 the " variation " over its area of distribution, and of the conditions under 

 which it lives in this range, I believe that I have a proper basis for an investiga- 

 tion of evolution, experimentally. The actual materials used in the investiga- 

 tion have in all instances been taken from some accessible location, not likely 

 to be disturbed by agricultural or other economic operations, and during the 

 course of the experiments, or at least during the crucial portion thereof, the 

 exact locality from which the material came has been visited at least twice in 

 each year at a time when the population could be seen in numbers, and at that 

 time a census was made to determine the presence in the population of any 

 divergent or modified traits or characters that were not found in the first 

 general census of the species. This hnowledge of the status of the materials in 

 nature at the point of origin of the experimental material is of the greatest 

 importance, because if any, modifications are produced in experiment, and if 

 these are new or striking it is then necessary to know positively whether the new 

 character ivas produced in experiment alone. 



I have been exacting upon this requirement in the character of my materials 

 for the reason that the bane of experimental evolution at the present time lies 

 in the unfortunate fact that all too often, as in De Vries's Oenotheras they were 

 " mutating " when he found them, and everyone is now at liberty to " interpret " 

 the results of De Vries as his own biological orthodoxy dictates. The rigid con- 

 ditions demanded in physical science should be for the biologist the model of 

 operation, a most rigorous study of the materials in nature, purification and 

 standardization, and then experimental operations with the proper setting to 

 give exact answers to specific problems. 



CHARACTER AND SOURCE OF MATERIALS. 

 THE LINEATA GROUP OF LEPTINOTARSAS. 



The animals which have furnished the materials upon which this report is 

 based are a homogeneous group of species — the lineata group — of the genus 

 Leptinotarsa. These organisms are confined almost entirely to North America, 

 3 



