vii ON THE INFERTILITY OF CROSSES 169 



which it is correlated ; and though these differences are 

 sometimes slight, and the amount of the infertility is not 

 always, or even usually, proportionate to the external dif- 

 ference between the two forms crossed, we must believe that 

 there is some connection between the two classes of facts. 

 This is especially the case as regards colour ; and Mr. Darwin 

 has collected a body of facts which go far to prove that 

 colour, instead of being an altogether trifling and un- 

 important character, as was supposed by the older natural- 

 ists, is really one of great significance, since it is un- 

 doubtedly often correlated with important constitutional 

 differences. Now colour is one of the characters that most 

 usually distinguishes closely allied species ; and when we 

 hear that the most closely allied species of plants are 

 infertile together, while those more remote are fertile, the 

 meaning usually is that the former differ chiefly in the colour 

 of their flowers, while the latter differ in the form of the 

 flowers or foliage, in habit, or in other structural characters. 



It is therefore a most curious and suggestive fact, that in 

 all the recorded cases, in which a decided infertility occurs 

 between varieties of the same species, those varieties are 

 distinguished by a difference of colour. The infertile 

 varieties of Verbascum were Avhite and yellow flowered 

 respectively ; the infertile varieties of maize were red and 

 yellow seeded; while the infertile pimpernels were the red 

 and the blue flowered varieties. So, the differently coloured 

 varieties of hollyhocks, though grown close together, each 

 reproduce their own colour from seed, showing that they are 

 not capable of freely intercrossing. Yet Mr. Darwin assures 

 us that the agency of bees is necessary to carry the pollen 

 from one plant to another, because in each flower the pollen 

 is shed before the stigma is ready to receive it. We have 

 here, therefore, either almost complete sterility between 

 varieties of different colours, or a prepotent effect of pollen 

 from a flower of the same colour, bringing about . the same 

 result. 



Similar phenomena have not been recorded among 

 animals ; but this is not to be wondered at when we consider 

 that most of our pure and valued domestic breeds are 

 characterised by definite colours which constitute one of their 



