Variation and Adaptation. 153 



A will have less difficulty in finding conditions to suit 

 it than a less variable one, such as might be described by 

 curve B. The offspring of seeds of varying parents are 

 therefore at a considerable advantage. 



And now we come to the significance of crossing. 

 The essence of fertilization is not the union of the two 

 sexes but the mixture of the heritable characters of two 

 individuals with a different past or at any rate of indi- 

 viduals which have been subjected to different external 

 conditions. The advantages accruing from the fusion 

 of different variants afford, in my opinion, a fair ex- 

 planation of the existence of sexual reproduction.^ 



Darwin's well-known aphorism : nature abhors per- 

 petual self-fertilization, does not seem to me to express 

 the matter quite exactly. It is not sufficient that isolated 

 crossings should occur from time to time ; on the con- 

 trary, it is necessary that a certain percentage of indi- 

 viduals should always be crossed. For in this way varia- 

 bility will be increased ;^ but the point is not that its range 

 should be as wide as possible but that it should be main- 

 tained at a limit which the environment demands.^ 



The degree of the deviation of the individual is al- 

 ready determined in the seed. But seeds differ among 

 themselves not only in relation to the characters of their 

 parents, but according to the position on the plant itself 

 and according to their weight. The significance of these 

 factors and their bearing on variability has often been 

 the subject of research ; numerous isolated papers on this 

 subject exist but they need a comparative and critical 



^ Intracelhdare Pangenesis, p. 29. 



^ A. GiARD in Comptcs rendus de la Soc. de Biologie, 4 Nov. 1899, 

 p. 2, and LiGNiER in Festschrift su Ehren Giard's, Nov. 1899. 



^ See especially Ammon, Der Ah'dnderungsspielraum, loc. cit., 

 P- 53. 



