The Paiiyciies. 647 



their number. Higher nutriticju and favorable e(^n(h- 

 tions of Hfe 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 (hlferent degree, and on the other 

 hand at different periods in the life of tlie ])lant. for ^onie 

 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 c(^nditions. 

 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 aft'ect the several 

 characters in a similar manner thoug^h 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. Tn 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, mav exist 

 in various conditions and ]^ositions. 1'he normal active 

 condition is that in which they multiply at a definite [K- 



