254 SUMMARY OF MEASUREMENTS. CHAP. VII. 



in fertility. It has, however, been shown that the 

 superiority of these intercrossed plants in height and 

 weight was in all probability not real ; for if the two 

 sets had been allowed to grow for another month, it is 

 almost certain that those from a cross with the fresh 

 stock would have been victorious in every way over 

 the intercrossed plants. 



Before we consider in detail the several cases given 

 in Table C, some preliminary remarks must be made. 

 There is the clearest evidence, as we shall presently 

 see, that the advantage of a cross depends wholly on 

 the plants differing somewhat in constitution ; and that 

 the disadvantages of self-fertilisation depend on the two 

 parents, which are combined in the same hermaphrodite 

 flower, having a closely similar constitution. A certain 

 amount of differentiation in the sexual elements seems 

 indispensable for the full fertility of the parents, and 

 for the full vigour of the offspring. All the individuals 

 of the same species, even those produced in a state of 

 nature, differ somewhat, though often very slightly, 

 from one another in external characters and probably in 

 constitution. This obviously holds good between the 

 varieties of the same species, as far as external characters 

 are concerned ; and much evidence could be advanced 

 with respect to their generally differing somewhat in 

 constitution. There can hardly be a doubt that the 

 differences of all kinds between the individuals and 

 varieties of the same species depend largely, and as I 

 believe exclusively, on their progenitors having been 

 subjected to different conditions ; though the conditions 

 to which the individuals of the same species are ex- 

 posed in a state of nature often falsely appear to us the 

 same. For instance, the individuals growing together 

 are necessarily exposed to the same climate, and they 

 seem to us at first sight to be subjected to identically 





