454 POPULAR SCIENCE MONTHLY. 



basis are combined, perhaps inextricably, but all attempts at assigning 

 them to a single cause or property have failed. 



Cross-fertilization is commonly misunderstood to be merely an ac- 

 cessory of reproduction, and a negative factor in evolution, because it 

 is supposed to conduce to the permanence of the specific type by aver- 

 aging away the new characters which arise as individual variations. 

 There is the amplest experimental evidence that cross-breeding is neces- 

 sary to maintain the quality and efficiency of the individual, but static 

 theories* require us to believe that evolutionary progress requires con- 

 ditions unfavorable to the individuals of which species are composed, 

 since under such conditions selection is most effective, and abrupt vari- 

 ations 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, as it were, the multiplier of the evolutionary equa- 

 tion, because it compels the distribution and combination of individual 

 variations into the resultant vital motion of the species. Evolution no 

 longer appears as an abnormal or exceptional phenomenon, and it be- 

 comes clear that the conditions under which the species is most pros- 

 perous are also those which permit the most rapid evolutionary progress. 



TJie Prepotency of Variations. 

 The first corollary of the law of symbasis is the prepotency of varia- 

 tions. The combination of variations not only permits the structure of 

 the organism to be strengthened and rendered more efficient, but also 

 gives prepotency, due to the opportunity of vital motion. Variant indi- 

 viduals being thus both vigorous and prepotent, it is easy to understand 

 why diversity, and not uniformity, is the tendency of normally ex- 

 tensive species; changes are necessary and welcome, and the perpetua- 

 tion of them does not require segregation. Numerous and well authen- 



*Static theories, under which species are held to be normally stationary, 

 may be subdivided into two groups, those which look upon evolutionary prog- 

 ress as gradual and 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 char- 

 acters,' or they may imply the notion of a somewhat constant range of varia- 

 bility in species, which are thought of as growing out farther on one side be- 

 cause selection keeps them pared off on the other. Movement is thus ascribed 

 variously to the direct action of the environment, to selective isolation, to 

 abrupt transformation or mutation, or to some combination of these. The 

 kinetic theory rejects all these supposed factors and interprets vital motion 

 as continuous, gradual and self-caused, or inherent in the species, but the 

 environment is thought of as influencing the direction of organic change. 

 Selective influence is neglected altogether by still other theories, such as that 

 of Naegeli, in which evolution is explained by an internal 'hereditary mech- 

 anism,' supposed to carry the species along in a definite direction. 



