THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 403 



determines the course of the stream, but tlie water descends by its 

 own gravity. 



In the course of its progress the species explores the adjacent ter- 

 ritory and follows the line of least resistance to the variations it is 

 able to put forth. Changes are necessary' to maintain the vitality of 

 the species and also to keep it abreast of its environmental oppor- 

 tunities, and if no adaptive movement can be made it is still unable 

 to remain stationary, but continues to change in characters indiffer- 

 ent to the environment, or even actually detrimental. 



The species encounters obstacles and subdivides l)ecause it is in 

 motion ; the diversity of form arises because variations can no longer 

 spread freely among the individuals of the species, not because the 

 environment introduces new characters. 



That species occupy definite regions of distribution has been taken 

 by some to mean that the individuals are similar because they are 

 molded by similar influences, but that this inference is wrong is 

 shown both by the wide diversity of conditions under Avhich some 

 species exist, and by the even wider diversity of form and structure 

 often found among the members of the same species, in the same en- 

 vironment. Similarity of conditions may permit plants and animals 

 of different origins to develop similar variations, and to share, 

 finally, the same adaptive characters, but identical conditions do not 

 put an end to individual variations, nor to evolutionary progress.'* 



THE INFLUENCE OF SELECTION. 



In denying that selection has any power to initiate or actuate 

 dcAelopmental changes there is no intention to im])ly that it has not 

 profoundly influenced the course of evolution in many groups. In- 

 deed, it may be claimed that the kinetic theory * afforded the first 

 concrete explanation of the workings of natural selection. Vital 

 motion not only makes selective influence possible, but it meets the 

 ancient and hitherto fatal objection to the doctrine of adaptation, 

 since it shows how characters may originate and develop to the point 

 of utility or harmfulness, where adaptive selection can take effect. 



That there are species, varieties, mutations, or hybrids which differ 

 in one, two, or three characters, as connnonly asserted in discussions 

 of Mendel's laws, is quite unwarranted by facts. The mention of a 

 single peculiar character may suffice to designate a species or variety 

 for taxonomic purposes, but in evolutionary studies it is careless to 



a " Even sugar beets, the oldest 'selected' agricultunil plants, are far from 

 having freed themselves from the necessity of continuous improvement. With- 

 out this they would not remain constant, but would retrograde with great 

 ral)iditJ^" DeVries, II., 190.5. Species and Varieties, 109. 



bCook, O. F., 1901. A. Kinetic Theory of Evolution, Science, N. S., 13:909. 



