450 POPULAR SCIENCE MONTHLY. 



Indeed, it may be claimed that the kinetic theory of evolution affords 

 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 theory of selection, 

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

 of utility or harmfulness, where selection can take effect. 



The hypothesis of selection as the active principle or causal agency 

 of evolution became illogical and useless as soon as the inheritance of 

 acquired characters was abandoned. The first idea without the second 

 does not account for adaptations. The 'selection' of Nageli, Weis- 

 mann and other believers in a 'determining principle' or 'hereditary 

 mechanism' of evolution is a very weak substitute for the Darwinian 

 idea, able only to eliminate the hopelessly unfit, but quite without 

 means of influencing the survivors. The recognition of a continuous 

 and necessary vital motion permits us to understand that the rejection 

 by the environment of a harmful variation encourages adaptation by 

 accelerating the development of any more adaptive variation which 

 may appear. 



All organisms are subject to selective influence in the sense that 

 variations are rejected with a promptness proportional to their harm- 

 fulness in the given environment, but generally this leaves a very wide 

 latitude of possible changes in which selection does not interfere. The 

 instances are relatively rare in which existence becomes acutely de- 

 pendent upon the development of some one characteristic or quality,, 

 and such narrow selection does not strengthen the type, but insures 

 and even hastens its extinction.* 



The Significance of Species. 



The traditional illustration of organic descent by a tree with ever- 

 dividing branches is entirely misleading as a suggestion of the nature 

 of evolutionary processes, because individuals do not follow each other 

 in simple series. Successive generations are connected by endless 

 intergraftings of the lines of descent. A species may be treated sys- 

 tematically or statistically as an aggregation of individuals, and may 

 be described by an averaging of the characters of these; but from an 

 evolutionary point of view it does not exist as a species because of the 

 possession of a certain complex of characters, but because the com- 

 ponent individuals breed together; through this alone is the integrity 

 or coherence of the species maintained. For evolutionary purposes 

 we may think of the san^e species existing thousands of years hence, 



* ' Stages of Vital Motion,' Poptjlak Science Monthly, LXIII., 16, May,. 

 1903. 



