ASPECTS OF KINETIC EVOLUTION 305 



explanation of variation, but contributes to such an achievement 

 by rendering the problem more definite. It affords another 

 conception of how evolution may be accomplished, but a con- 

 ception more comprehensive than those which have gone before ; 

 one which does not depend upon any theoretical or doubtful 

 relation, but upon the well ascertained and universal fact that 

 organisms exist everywhere in species — groups of diverse indi- 

 viduals freely interbreeding to form a complex network or fabric 

 of descent. 



To some there may appear to be no practical distinction 

 between the static and the kinetic views. Not a few naturalists 

 have entertained truly kinetic conceptions of the facts of organic 

 nature, even while continuing to misrepresent them by the use 

 of the static terminology. For descriptive purposes, such as the 

 tracing of phylogenies, the differences are less important, but 

 fundamental divergence is obvious in approaching the physio- 

 logical questions of methods and causes. The probable truth of 

 a theory does not depend merely upon the number of facts which 

 can be assembled under it, but also upon the coherence and 

 practical consistency of the relations alleged. Of two theories 

 otherwise equal the more simple and direct should receive the 

 greater confidence. The kinetic theory is not compelled to 

 ascribe utility to all characters, and can explain useful and use- 

 less characters by reference to the same facts of organic diversity 

 and association in species. 



SUMMARY OF EVOLUTION THEORIES. 



Static theories view the species as normally stationary, and 

 ascribe evolutionary motion to environmental causes of adapta- 

 tion. The static theory commonly called Darwinism (though 

 avoided by Darwin himself) treats adaptations as caused indi- 

 rectly through natural selection, by the survival of the fittest of 

 the individual variations. The static theory of Lamarckism 

 treats adaptations as direct results or responses to environmental 

 influences. 



Saltatory theories view the species as normally stationary 

 except for rare intervals of sudden transformation or " muta- 

 tion " caused either by the environment or by internal "forces" 



