It is a more difficult problem why some plants and apparently all the higher animals, after becoming hermaphrodites, have since had their sexes re-separated.  This separation has been attributed by some naturalists 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 organisation; and that at the same time there was no need for all the individuals to produce young, and consequently 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 crossed species are subjected.  The pollen of each species is strongly prepotent over that of any other species, so that if a plant’s own pollen is placed on the stigma some time after foreign pollen has been applied to it, any effect from the latter is quite obliterated.  It is also notorious that not only the parent species, but the hybrids raised from them are more or less sterile; and that their pollen is often in a more or less aborted condition.  The degree of sterility of various hybrids does not always strictly correspond with the degree of difficulty in uniting the parent forms.  When hybrids are capable of breeding inter se, their descendants are more or less sterile, and they often become still more sterile in the later generations; but then close interbreeding has hitherto been practised in all such cases.  The more sterile hybrids are sometimes much dwarfed in stature, and have a feeble constitution.  Other facts could be given, but these will suffice for us.  Naturalists formerly attributed all these results to the difference between species being fundamentally distinct from that between the varieties of the same species; and this is still the verdict of some naturalists.