INTENSIVE SEGBEOATION. 321 



less divergent evolution is the laiv of Intensive Segregation already 

 referred to ; but in addition to this certainty tliere is a very 

 strong probability that whore Independent Generation is long 

 continued, transformation of some kind will supervene. If there 

 are any species in which the power of cumulative variation has 

 been entireh'^ lost, this latter law cannot .hold in their case ; but 

 it is doubtful whether among species that reproduce sexually 

 there are many such. The variability of some species is so smnll, 

 and the conditions of the environment are so constant, that com- 

 paratively long periods of Independent Greneration pass before 

 perceptible transformation arises. This seems to be the case 

 with the 13- and 17-year races of Cicada septemdecim, to which I 

 shall refer when giving examples from nature. From the high 

 probability that long-continued Independent G-eneration will be 

 followed by Independent Transformation, and the certainty that 

 Independent Transformation will be divergent, there follows the 

 corollary that long-continued Independent Generation will pro- 

 bably be attended by divergence. In other words, Independent 

 Generation long continued is almost always attended by Inde- 

 pendent Transformation ; and Independent Transformation in- 

 evitably produces Divergence. This double principle I call the 

 law of Intension. This law rests on the ubiquity of transforming 

 influence, and on the impossibility that in a species possessing any 

 plasticity the inherited effects in any section independently 

 generating should be exactly the same as in any other section. 



We cannot doubt that, when a diversity of powers and sus- 

 ceptibilities in the different sections is acted upon by a great 

 variety of influences, the responses of the different sections will 

 be unlike ; and the result will be increasing segregation and 

 increasing divergence. Now it is impossible to doubt that in 

 species propagating sexually, and possessing some degree of plas- 

 ticity, these are exactly the conditions whenever the species is 

 divided into sections that do not intergenerate. 



It should be observed that, in accordance with the principle of 

 Intension, not only is indiscriminate Separate G-eneration when 

 long continued transformed into more and more strongly Segre- 

 gate Generation, but any form of Segregate Generation, resting 

 on some one principle that causes the division of the species into 

 sections difleritig in regard to some one form of endowment, 

 will, if long continued, be inevitably reinforced and intensified by 

 transformations, which, being independently combined and trans- 



