THE LAW OF INTENSION. 193 



been entirely lost, this latter law can not 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 small, and the con- 

 ditions of the environment are so constant, that comparatively long 

 periods of independent generation pass before perceptible transfor- 

 mation arises. This seems to be the case with the thirteen and 

 seventeen year races of Cicada septendecim, to which I shall refer when 

 giving examples from nature. From the high probability that long- 

 continued independent generation (i. e., isolation) 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 probably be attended by 

 divergence. In other words, independent generation long continued 

 is almost always attended by independent transformation ; and inde- 

 pendent transformation inevitably 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 indepen- 

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

 This is especially the case when the species is highly plastic and when 

 the isolated section is very small. 



We can not doubt that when a diversity of powers and suscepti- 

 bilities 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 plasticity, these are exactly the con- 

 ditions whenever the species is divided into sections that do not inter- 

 generate. 



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

 intension, not only is indiscriminate separate generation when long- 

 continued transformed into more and more strongly segregate gen- 

 eration, but any form of segregate generation, resting on some one 

 principle that causes the division of the species into sections differing 

 in regard to some one form of endowment, will, if it is long continued, 

 be inevitably reinforced and intensified by transformations, which, 

 being independently combined and transmitted, will multiply the 

 number of characteristics in regard to which divergence takes place. 

 If, for example, the pollen of a given variety, when falling upon the 

 stigma of the same variety or race, is prepotent over the pollen of 

 every other variety or race that falls Upon the same stigma at the 



