INTRODUCTION. 23 



between organs or parts offers one of the most promising of objectives and one that may 

 serve to explain structures whose origin is now entirely obscure. It seems probable 

 that the correlation between parts is at bottom chiefly a question of competition for food, 

 and, if this be true, the widest experimental vista is opened. While any of these 

 problems may serve as the entry to experimental evolution, they are all so interrelated 

 that a comprehensive attack upon them alone promises to yield adequate results in a 

 reasonable time. 



Variads. — The ecological attack upon the problems of evolution has led to the emphasis 

 of three fundamental points, namely, origin, fixation, and differentiation. It proceeds 

 upon the certain knowledge that new forms of plants are constantly being produced 

 by the impact of environment, and hence its first task is to analyze the various methods 

 of origin (Clements, 1907, 1908). Further studies of origin at the Alpine Laboratory 

 have confirmed the hypothesis that direct adaptation to the habitat has there produced 

 the largest number of new forms of plastic species. Mutation now seems less important 

 than it did 15 years ago, but this is partly due to the increasing difficulty of distinguishing 

 mutants from variants, and sometimes even from ecads. Much of variation is undoubt- 

 edly response to the gradual change of an efficient factor, or to minute habitats of varying 

 intensity. Further search has not increased the small number of probable hybrids, and 

 it has become necessary to attempt the direct production of hybrids in nature. In 

 spite of the changing importance of the methods of origin, it still appears certain that 

 adaptation, mutation, variation, and hybridization comprise the four processes of 

 evolution, though it now seems evident that adaptation and hybridization constitute 

 the two basic modes. The resulting differentiations of the species are distinguished 

 as ecads, mutants, variants, and hybrids, and are included under the general term 

 variad. As to fixation of these, evidence is slowly accumulating to show that this may 

 be cumulative, and that even ecads may run the whole gamut of constancy from the 

 most inconstant to the most fixed. Moreover, one character, such as form, may become 

 fixed, while another arising from the same factor, such as the time of blooming, may be 

 unstable. Finally, it seems clear to the ecologist that degree of differentiation must 

 come to play a more definite role in the question of species and variads, and that this 

 can be attained only through objective measurement. 



METHODS OF TREATMENT. 



General plan. — On the basis of the principles discussed above, a series of monographs 

 is planned to comprise a large number of the most important genera and families of 

 North America. For a number of reasons the interest centers in the West, chiefly because 

 this still affords the fullest opportunity for the application of statistical and experimental 

 methods on a large scale. It provides a much greater range of climatic and edaphic 

 conditions, and to this appears to be related a correspondingly greater evolutionary 

 activity. A further reason of the greatest importance hes in the fact that the four 

 bases for ecological research, namely. Pike's Peak, Tucson, Berkeley, and Lincoln, 

 afford unique opportunity for applying the phylogenetic method in the midst of the 

 great cHmates and climaxes of the West. Finally, the vegetation of the West is not 

 only of the first importance in relation to forestry, grazing, agriculture, land classification, 

 and utilization, but it probably also furnishes the best opportunity in the world for the 

 comprehensive and fundamental development of the science of vegetation itself in relation 

 to cUmate and soil. 



The three genera treated in this monograph were selected because of their taxonomic 

 and ecologic interest. They have been objects of the most active evolution, with which 

 has gone the widest distribution, both as to cUmate and soil. Ecologically, they are 

 unsurpassed in the number of dominants and subdominants they furnish to the vegeta- 



