Chap. XII. GENERAL KESULTS. 461 



We thus have a long series with absolute sterility at 

 the two ends ; at one end due to the sexual elements 

 not having been sufficiently differentiated, and at the 

 other end to their having been differentiated in too 

 great a degree, or in some peculiar manner. 



The fertilisation of one of the higher plants depends, 

 in the first place, on the mutual action of the pollen- 

 grains and the stigmatic secretion or tissues, and after- 

 wards on the mutual action of the contents of the 

 pollen-grains and ovules. Both actions, judging from 

 the increased fertility of the parent-plants and from the 

 increased powers of growth in the offspring, are favoured 

 by some degree of differentiation in the elements 

 which interact and unite so as to form a new being. 

 Here we have some analogy with chemical affinity or 

 attraction, which comes into play only between atoms 

 or molecules of a different nature. As Prof. Miller 

 remarks : " Generally speaking, the greater the dif- 

 ference in the properties of two bodies, the more intense 

 is their tendency to mutual chemical action. . . . But 

 between bodies of a similar character the tendency to 

 unite is feeble."* This latter proposition accords well 

 with the feeble effects of a plant's own pollen on the 

 fertility of the mother-plant and on the growth of the 

 offspring ; and the former proposition accords well with 

 the powerful influence in both ways of pollen from an 

 individual which has been differentiated by exposure 

 to changed conditions, or by so-called spontaneous 

 variation. But the analogy fails when we turn to the 

 negative or weak effects of pollen from one species on 

 a distinct species ; for although some substances which 

 are extremely dissimilar, for instance, carbon and 



* 'Elements of Chemistry,' 4th views with respect to chemical 

 edit. 1867, part i. p. 11. Dr. affinity are general iv accepted by 

 Frankland informs me that similar chemists. 



