THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 399 



tage for the menihers of tlie species to become more Jind more diverse, 

 not in order to subdivide into more numerous species, but because 

 diversity of descent increases the vigor of the individual organisms 

 of which the species is composed. 



Sexual and other diversification inside the species has continued to 

 l:)ecome more and more accentuated in hundreds of indej^endent 

 groups of plants and animals, and is ever3'Avhere recognized as a 

 mark of greater organic perfection. Professor Weismann held that 

 under constant external influences variation would not occur;" but 

 the practical fact is that in the same species, and under the same 

 general environments, variations not only occur, l)ut are preserved, 

 accumulated, and integrated into sexual dift'erences, not by isolating 

 a part of the species from the rest, but under conditions of free and 

 continuous interbreeding. The differences between the sexes are 

 commonly greater than those which separate the species and genera, 

 or even than those which characterize the families and orders, show- 

 ing most conclusively that such dift'erences can arise and become 

 established, even inside the species, and quite without segregation. 

 But instead of having been appreciated as tlie most important 

 agency and the most significant illustration of evolution, sex and 

 symbasic interbreeding have continued to be regarded as obstacles, 

 because they interfere with the fancied necessity of segregation. 



Evolutionists, too intent on a practical explanation of the diversity 

 of species, magnified the idea that orgatiisms become adapted to 

 environment, and disregarded tlie more fundamental fact that spe- 

 cies are not by nature stationary, but liave an independent motion of 

 their own. This oversight brought us the impossible task of explain- 

 ing how external conditions produce evolutionary changes, and 

 prevented the ]x>rce])tion that adaptations are due to external causes 

 only as environment may influence the direction of the normal and 

 necessary movement of the species. 



That evolution is thus an active, universal, and truly physiological 

 process is not considered in current theories, largely because thought 

 and language have continued to follow the bias of the original Dar- 

 winian controversy in seeking in evolution an explanation of the 

 origin of species, and in expecting, conversely, that an explanation of 

 the origin of species would also explain evolution. Such a history 

 greatly increases the difficulty of presenting this alternative view, that 

 the multiplication of species is in no pro2)er sense a result of evolution, 

 but is due to entirely distinct causes more often antagonistic than 

 favorable to evolutionary progress. 



« Weismann, A., 1893, The Germ Plasm, JCo. 



