EVOLUTION NOT THE ORIGIN OF SPECIES. 449 



physical barriers in the work of subdividing species, this would mean 

 that evolution sometimes results in segregation, not that segregation 

 results in evolution. 



Evolution is a process of change in species; it is the journey of 

 which individual variations are steps. Evolution changes the charac- 

 ters of species, but it does not originate species. 



Natural selection may assist geographical and other influences tend- 

 ing to the division of species, but it is not on that account a cause of 

 evolution; it represents the determining aspect of the environment — 

 the factors which influence the direction of the vital motion, but not 

 those which induce the motion. Natural selection may explain differ- 

 ences between two species, but not the becoming different. It is an 

 external incident or influence and not an active principle or agency of 

 organic evolution. Adaptation is possible because there is a vital 

 motion which can be deflected, not because the environment changes 

 the characters of species. The river of evolution flows through the 

 land of environment; the conformation of the valley determines the 

 course of the stream, but the water descends by its own gravity. 



In the course of its progress the species explores all the adjacent 

 territory 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 opportuni- 

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

 remain stationary, but continues to change in characters indifferent to 

 the environment, or even actually detrimental. 



The species encounters obstacles and subdivides because it is in 

 motion; the division takes place when variations can no longer spread 

 freely among the individuals of the species, not because the environ- 

 ment 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 vdde diversity of conditions under which 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 environment. 

 Similarity of conditions may permit plants and animals of different 

 origins to develop similar variations, and to share, flnally, the same 

 adaptive characters, but identical conditions do not put an end to indi- 

 vidual variations or to evolutionary progress. 



The Function of Selection. 



By denying that selection has any power to initiate or actuate de- 

 velopmental changes it is not intended to imply that it has not pro- 

 foundly influenced the course of evolution in many organic groups. 



VOL. Lurv. — 29. 



