The Essence of Fertilisation 31 



single cross, however, of such a colony with individuals 

 of another origin restored the original vigor. 



The process of fertilization, in its essence, does not 

 consist, therefore, in the union of two sexes, but in the 

 mixing of the hereditary characters of two individuals of 

 different origin, or at least of such as have been subjected 

 to different external conditions. Therefore, a difference 

 in hereditary characters is obviously a condition for at- 

 taining the full advantage of fertilization ; this difference, 

 however, must have been acquired in the last instance 

 through a life under different influences. 



Let us regard the individual hereditary factors as in- 

 dependent units, which can be combined with each other 

 in different proportions into the individual character of a 

 plant. Let us further assume that their relative increase 

 or decrease depends on external influences. Evidently 

 there is then a great probability that, under similar ex- 

 ternal conditions, the same factors will deteriorate in 

 different individuals, while under different conditions this 

 fate will befall other factors in every individual. Thus, 

 on crossing the plants of the same bed only, the individual 

 deviations of the same kind are strengthened; the weak- 

 ened factors are therefore made still weaker. But if we 

 cross individuals of the most different culture possible, 

 the differences in the individual factors are clearly bal- 

 anced, at least in part; and this the more so, the more 

 numerous the specimens which deviate from each other, 

 and which are used for the crossing. 



It is well known to plant breeders that luxurious con- 

 ditions which are varied as much as possible lead to an 

 accumulation and increase of individual differences, while 

 simple and uniform circumstances make them disappear 

 gradually, and thus further the uniformity of all speci- 



