308 Nutrition and Selection. 



and, to a no less extent, on nutrition. In the third part 

 of the first vohime, the curve representing the length of 

 tlie fruits in OcnotJicra 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 

 hulhosus, 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. 



