CHAP. XII. GENERAL RESULTS. 447 



that allied forms should exhibit an extreme diversity 

 in the means which favour either end. If, as there is 

 reason at least to suspect, self-fertilisation is in some 

 respects beneficial, although more than counterbalanced 

 by the advantages derived from a cross with a fresh 

 stock, the problem becomes still more complicated. 



As I only twice experimented on more than a single 

 species in a genus, I cannot say whether the crossed 

 offspring of the several species within the same genus 

 differ in their degree of superiority over their self- 

 fertilised brethren ; but I should expect that this 

 would often prove to be the case from what was 

 observed with the two species of Lobelia and with the 

 individuals of the same species of Nicotiana. The 

 species belonging to distinct genera in the same family 

 certainly differ in this respect. The effects of cross- 

 and self-fertilisation may be confined either to the 

 growth or to the fertility of the offspring, but generally 

 extends to both qualities. There does not seem to 

 exist any close correspondence between the degree to 

 which the flowers of species are adapted for cross- 

 fertilisation, and the degree to which their offspring 

 profit by this process ; but we may -easily err on this 

 head, as there are two means for favouring cross-ferti- 

 lisation which are not externally perceptible, namely, , 

 self-sterility and the prepotent fertilising influence of 

 pollen from another individual. Lastly, it has been 

 shown in a former chapter that the effect produced by 

 cross and self-fertilisation on the fertility of the parent- 

 plants does not always correspond with that produced 

 on the height, vigour, and fertility of their offspring. 

 The same remark applies to crossed and self-fertilised 

 seedlings when these are used as the parent-plants. This 

 want of correspondence probably depends, at least in 

 part, on the number of seeds produced being chiefly 



