308 Nutrition and Selection. 
and, to a no less extent, on nutrition. In the third part 
of the first volume, the curve representing the length of 
the fruits in Ocnothcra and the curves of the rays of cer- 
tain Umbelli ferae and Compositae also proved that these 
two factors operate in the same direction. Active and 
semi-latent characters are thus shown to behave in the 
same way with regard to these two factors. 
Since, however, the extraordinary variability of semi- 
latent characters (of which an account has been given 
in 2 of this part, p. 9), is one of the strongest supports 
of the doctrine of selection, it seems to me that it is worth 
while to attempt to make the relation between this phe- 
nomenon and nutrition specially clear. In this last chap- 
ter I will therefore deal with a series of facts gathered 
partly from the literature of the subject and partly from 
my own observations, which all point more or less defi- 
nitely to the conclusion that semi-latent characters are 
largely dependent on the external conditions of life. 
External influences exert their effect on the develop- 
ment of organs during their youth, that is to say during 
the so-called susceptible period. After the character of 
the organ has been definitely established in this period, 
the further development cannot affect it. The number 
of leaflets in a clover leaf, of the petals of Ranunculus 
bulbosus, of the accessory carpels of Papaver are finally 
determined in this period; but the conditions of life at 
the critical moment are not the only factor. The accu- 
mulated effects of previous influences have also com- 
bined to determine the individual strength of the organ 
or of the individual ; and the part which this latter factor 
plays in the determination of the degree of development 
of the deviating character is sometimes greater and some- 
times less than that of the immediate external conditions. 
