468 GENERAL RESULTS. CUAP. XII. 



It is a more difficult problem why some plants and 

 apparently all the higher animals, after becoming her- 

 maphrodites, have since had their sexes re-separated. 

 This separation has been attributed by some natural- 

 ists to the advantages which follow from a division of 

 physiological labour. The principle is intelligible 

 when the same organ has to perform at the same time 

 diverse functions ; but it is not obvious why the male 

 and female glands when placed in different parts of 

 the same compound or simple individual, should not 

 perform their functions equally well as when placed in 

 two distinct individuals. In some instances the sexes 

 may have been re-separated for the sake of preventing 

 too frequent self-fertilisation; but this explanation 

 does not seem probable, as the same end might have 

 been gained by other and simpler means, for instance 

 dichogamy. It may be that the production of the 

 male and female reproductive elements and the 

 maturation of the ovules was too great a strain and 

 expenditure of vital force for a single individual to 

 withstand, if endowed with a highly complex organi- 

 sation ; and that at the same time there was no need 

 for all the individuals to produce young, and conse- 

 quently that no injury, on the contrary, good resulted 

 from half of them, or the males, failing to produce 

 offspring. 



There is another subject on which some light is 

 thrown by the facts given in this volume, namely, 

 hybridisation. It is notorious that when distinct 

 species of plants are crossed, they produce with the 

 rarest exceptions fewer seeds than the normal number. 

 This unproductiveness varies in different species up to 

 sterility so complete that not even an empty capsule 

 is formed ; and all experimentalists have found that 

 it is much influenced by the conditions to which the 



