The Pangcnes. 647 
their number. Higher nutrition and favorable condi- 
tions of life effect an increase, whereas the opposite cir- 
cumstances cause a decrease of this number. But the 
various kinds of pangenes are susceptible to these stimuli, 
on the one hand in a different degree, and on the other 
hand at different periods in the life of the plant, for some 
characters are highly variable, others not at all. In the 
first volume we have pointed out the existence of sus- 
ceptible periods of variability. They teach us how it is 
possible that the different characters of organisms may 
react in different ways to the same external conditions. 
Correlative variability, in so far as it is not due to a 
coupling of pangenes by their association in groups, finds 
its sufficient explanation in this way. 
The significance of normal fertilization appears in 
quite a new light when viewed from the standpoint of 
this conception. The conditions of life affect the several 
characters in a similar manner though in a different de- 
gree ; but they cannot, so far as we can judge at present, 
combine in the same individual characters, which deviate 
in opposite directions. The only practical way in which 
this can be effected is by an exchange of elements, such 
as happens in fertilization and probably especially at the 
beginning of the formation of the sexual cells. In this 
way sexual reproduction can unite characters which vary 
in different degrees and directions, in every possible kind 
of combination; and it is left to natural selection to de- 
cide which of these combinations are the best in every 
individual case. 
The theory of mutation assumes that the pangenes, or 
groups of similar pangenes in the idioplasm, may exist 
in various conditions and positions. The normal active 
condition is that in which they multiply at a definite pe- 
