410 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



individual, but static theories « have led to the belief that evolutionary 

 i^rogress requires conditions unfavorable to the individuals of which 

 species are composed, since under such conditions selection is most 

 effective and abrupt variations are most striking and numerous. The 

 alternative kinetic theory holds that cross-fertilization, as the active 

 agency of symbasis, is a positive and primary factor of evolution, 

 coordinate with variation itself. Symbasis is the multiplier of the 

 evolutionary equation ; it brings about the distribution and com- 

 bination of individual variations into the resultant vital motion of 

 the species. Evolution no longer appears as an abnormal or excep- 

 tional phenomenon, and it becomes clear that the conditions under 

 which the species is most prosperous are also those which permit the 

 most rapid evolutionary progress. 



THE PREPOTENCY OF VARIATIONS. 



The first corrollary of the law of symbasis is the prepotency of 

 variations. The combination of variations not only permits the struc- 

 ture of the organism to be strengthened and rendered more efficient, 

 but also gives prepotency, due to the opportunity of vital motion. 

 Variant individuals being thus both vigorous and prepotent, it is easy 

 to understand why diversity, and not uniformity, is the tendency of 

 normally extensive species; changes are necessary and welcome, and 

 the perpetuation of them does not require segregation. Numerous 

 and well authenticated instances of distinctly prepotent variations 



o static theories, under which species are held to be normally stationary, may 

 he subdivided into two groups, those which look upon evolutionary progress as 

 gradual and as actuated or carried along by natural selection, and those which 

 treat the motion as discontinuous or saltatory and due, not to selection, but 

 to abrupt variation or mutation. Selective theories, again, may hold either 

 that the environment causes the desirable variations or " acquired characters." 

 or they may imply the motion of a somewhat constant range of variability in 

 species, which are thought of as growing out farther on one side because selec- 

 tion keeps them pared off on the other. Movement is thus ascribed variously to 

 the direct action of the environment, to selective isolation, to abrupt trans- 

 formation or nmtation, or to some combination of these. 



Tinder Ilaeckel's biogenetic law evolution ai)pears as a resultant of (1) 

 palivf/cncsi.'i, a positive hereditary tendency to repeat the ancestral form, and 

 (2) kniogcnrsis, a negative, disturi)ing, adaptive influence located in the environ- 

 ment. The late Professor Hyatt sununarized a similar view by characterizing 

 heredity as (/cntipetal and environment as gcntifiigal, the one tending to make 

 all individuals alike, the other causing difference and evolutionary progress. 



The kinetic theory depends upon none of these supposed factors, but inter- 

 prets vital motion as continuous, gradual, and self-caused, or Inherent in the 

 si)ecies, though the environment is thought of as intluencing the direction of 

 organic change. Selective influence is neglected altogether by still other 

 theories, such as that of Naegeli, in which evolution is expl lined by an internal 

 " hereditary mechanism," supposed to carry the species along in a definite direc- 

 tion. 



