With respect to the influence of the conditions of life on any two breeds which are allowed to cross freely, unless both are indigenous and have long been accustomed to the country where they live, they will, in all probability, be unequally affected by the conditions, and this will modify the result. Even with indigenous breeds, it will rarely or never occur that both are equally well adapted to the surrounding circumstances; more especially when permitted to roam freely, and not carefully tended, as will generally be the case with breeds allowed to cross. As a consequence of this, natural selection will to a certain extent come into action, and the best fitted will survive, and this will aid in determining the ultimate character of the commingled body.
How long a time it would require before such a crossed body of animals would assume within a limited area a uniform character no one can say; that they would ultimately become uniform from free intercrossing, and from the survival of the fittest, we may feel assured; but the character thus acquired would rarely or never, as we may infer from the several previous { 90} considerations, be exactly intermediate between that of the two parent-breeds. With respect to the very slight differences by which the individuals of the same sub-variety, or even of allied varieties, are characterised, it is obvious that free crossing would soon obliterate such small distinctions. The formation of new varieties, independently of selection, would also thus be prevented; except when the same variation continually recurred from the action of some strongly predisposing cause. Hence we may conclude that free crossing has in all cases played an important part in giving to all the members of the same domestic race, and of the same natural species, uniformity of character, though largely modified by natural selection and by the direct action of the surrounding conditions.
_On the possibility of all organic beings occasionally intercrossing._--But it may be asked, can free crossing occur with hermaphrodite animals and plants? All the higher animals, and the few insects which have been domesticated, have separated sexes, and must inevitably unite for each birth. With respect to the crossing of hermaphrodites, the subject is too large for the present volume, and will be more properly treated in a succeeding work. In my 'Origin of Species,' however, I have given a short abstract of the reasons which induce me to believe that all organic beings occasionally cross, though perhaps in some cases only at long intervals of time.[192] I will here just recall the fact that many plants, though hermaphrodite in structure, are unisexual in function;--such as those called by C. K. Sprengel _dichogamous_, in which the pollen and stigma of the same flower are matured at different periods; or those called by me _reciprocally dimorphic_, in which the flower's own pollen is not fitted to fertilise its own stigma; or again, the many kinds in which curious mechanical contrivances exist, effectually preventing self-fertilisation. There are, however, many hermaphrodite plants which are not in any way specially constructed to favour intercrossing, but which nevertheless commingle almost as freely as animals with separated sexes. This is the case with cabbages, radishes, and onions, as I know from { 91} having experimented on them: even the peasants of Liguria say that cabbages must be prevented "from falling in love" with each other. In the orange tribe, Gallesio[193] remarks that the amelioration of the various kinds is checked by their continual and almost regular crossing. So it is with numerous other plants.