East and Jones in their recent valuable survey of our present knowledge on the subject[252] conclude quite definitely that inbreeding is not in itself productive of ill effects, the results of inbreeding in any particular case depending entirely upon the hereditary qualities transmitted; so that, although in bad stock the intensification of undesirable qualities through inbreeding might soon bring about deterioration, in good stock inbreeding is the surest method of making the desirable qualities a stable and permanent characteristic of the race. Nevertheless there are, in the opinion of these writers, certain advantages of a general nature to be derived from outbreeding connected principally: (1) with the occurrence of heterosis or hybrid vigour as a result of outbreeding, (2) with the fact that outbreeding leads to greater variability between individuals than does inbreeding, thus giving greater scope for the action of selective agencies and therefore endowing the race with greater power of adaptation to a changing environment--a factor which is probably of very considerable importance and which indeed seems to have been overlooked in a number of previous discussions of the subject, especially by nonbiological writers. [Sidenote: and can scarcely account for the intensity of the incest prohibition] It thus appears that stress should be laid upon the advantages of outbreeding rather than upon the supposed ill effects of inbreeding. Nevertheless we must admit that there exist biological factors of such a kind as to be capable of influencing the psychological attitude towards incest in the way that Westermarck's theory requires. But although the theory that incest prohibition is due to natural selection working on the _relative_ disadvantages of inbreeding may be correct so far as it goes, this does not absolve us from the duty of looking elsewhere for other factors which may have worked in the same direction. For it appears very doubtful whether the factors we have just been considering can be regarded as an adequate explanation of incest prohibition as we know it. If it is the advantages of outbreeding rather than the disadvantages of inbreeding that are potent; if the evil effects of inbreeding are so relatively slight as to leave room for doubt as to their nature and even the fact of their existence; if they are of such kind as to leave healthy stocks but little if at all affected and to become serious only after long continuance without admixture of fresh blood from outside (a state of affairs that can but rarely have occurred); and if they are liable to be counteracted by a change of locality or of life's conditions (which must sometimes have occurred, especially among nomadic peoples): then it is not easy to understand how such a widespread and powerful human characteristic as the aversion to incest can have arisen _solely_ as the result of natural selection, working through the bad effects of incest or the