THE LAW OF INTENSION. SS 
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 (7. 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 asin 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, itis 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 
