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occasional cross。 And； as in the case of flowers； I have as yet failed； after consultation with one of the highest authorities； namely； Professor Huxley； to discover a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the body； that access from without and the occasional influence of a distinct individual can be shown to be physically impossible。 Cirripedes long appeared to me to present a case of very great difficulty under this point of view； but I have been enabled； by a fortunate chance； elsewhere to prove that two individuals； though both are self…fertilising hermaphrodites； do sometimes cross。
It must have struck most naturalists as a strange anomaly that； in the case of both animals and plants； species of the same family and even of the same genus； though agreeing closely with each other in almost their whole organisation； yet are not rarely； some of them hermaphrodites； and some of them unisexual。 But if； in fact； all hermaphrodites do occasionally intercross with other individuals； the difference between hermaphrodites and unisexual species； as far as function is concerned； becomes very small。
From these several considerations and from the many special facts which I have collected； but which I am not here able to give； I am strongly inclined to suspect that； both in the vegetable and animal kingdoms； an occasional intercross with a distinct individual is a law of nature。 I am well aware that there are； on this view； many cases of difficulty； some of which I am trying to investigate。 Finally then； we may conclude that in many organic beings； a cross between two individuals is an obvious necessity for each birth； in many others it occurs perhaps only at long intervals； but in none； as I suspect； can self…fertilisation go on for perpetuity。
Circumstances favourable to Natural Selection
This is an extremely intricate subject。 A large amount of inheritable and diversified variability is favourable； but I believe mere individual differences suffice for the work。 A large number of individuals； by giving a better chance for the appearance within any given period of profitable variations； will compensate for a lesser amount of variability in each individual； and is； I believe； an extremely important element of success。 Though nature grants vast periods of time for the work of natural selection； she does not grant an indefinite period； for as all organic beings are striving； it may be said； to seize on each place in the economy of nature； if any one species does not become modified and improved in a corresponding degree with its competitors； it will soon be exterminated。
In man's methodical selection； a breeder selects for some definite object； and free intercrossing will wholly stop his work。 But when many men； without intending to alter the breed； have a nearly common standard of perfection； and all try to get and breed from the best animals； much improvement and modification surely but slowly follow from this unconscious process of selection； notwithstanding a large amount of crossing with inferior animals。 Thus it will be in nature； for within a confined area； with some place in its polity not so perfectly occupied as might be； natural selection will always tend to preserve all the individuals varying in the right direction； though in different degrees； so as better to fill up the unoccupied place。 But if the area be large； its several districts will almost certainly present different conditions of life； and then if natural selection be modifying and improving a species in the several districts； there will be intercrossing with the other individuals of the same species on the confines of each。 And in this case the effects of intercrossing can hardly be counterbalanced by natural selection always tending to modify all the individuals in each district in exactly the same manner to the conditions of each； for in a continuous area； the conditions will generally graduate away insensibly from one district to another。 The intercrossing will most affect those animals which unite for each birth； which wander much； and which do not breed at a very quick rate。 Hence in animals of this nature； for instance in birds； varieties will generally be confined to separated countries； and this I believe to be the case。 In hermaphrodite organisms which cross only occasionally； and likewise in animals which unite for each birth； but which wander little and which can increase at a very rapid rate； a new and improved variety might be quickly formed on any one spot； and might there maintain itself in a body； so that whatever intercrossing took place would be chiefly between the individuals of the same new variety。 A local variety when once thus formed might subsequently slowly spread to other districts。 On the above principle； nurserymen always prefer getting seed from a large body of plants of the same variety； as the chance of intercrossing with other varieties is thus lessened。
Even in the case of slow…breeding animals； which unite for each birth； we must not overrate the effects of intercrosses in retarding natural selection； for I can bring a considerable catalogue of facts； showing that within the same area； varieties of the same animal can long remain distinct； from haunting different stations； from breeding at slightly different seasons； or from varieties of the same kind preferring to pair together。
Intercrossing plays a very important part in nature in keeping the individuals of the same species； or of the same variety； true and uniform in character。 It will obviously thus act far more efficiently with those animals which unite for each birth； but I have already attempted to show that we have reason to believe that occasional intercrosses take place with all animals and with all plants。 Even if these take place only at long intervals； I am convinced that the young thus produced will gain so much in vigour and fertility over the offspring from long…continued self…fertilisation； that they will have a better chance of surviving and propagating their kind； and thus； in the long run； the influence of intercrosses； even at rare intervals； will be great。 If there exist organic beings which never intercross； uniformity of character can be retained amongst them； as long as their conditions of life remain the same； only through the principle of inheritance； and through natural selection destroying any which depart from the proper type； but if their conditions of life change and they undergo modification； uniformity of character can be given to their modified offspring； solely by natural selection preserving the same favourable variations。
Isolation； also； is an important element in the process of natural selection。 In a confined or isolated area； if not very large； the organic and inorganic conditions of life will generally be in a great degree uniform； so that natural selection will tend to modify all the individuals of a varying species throughout the area in the same manner in relation to the same conditions。 Intercrosses； also； with the individuals of the same species； which otherwise would have inhabited the surrounding and differently circumstanced districts； will be prevented。 But