As to the general existence of this horror there can be no doubt, and to assign to it an important part in framing human opinions and institutions with regard to incest is perfectly justifiable so long as we do not lose sight of the fact that this horror is only one side of the total human attitude towards the matter and that alongside of the horror there exists an attraction towards incest which corresponds in intensity to that of the horror itself[250]. An explanation in terms of this incest horror is not, however, that which we are seeking; it is, on the contrary, the very existence of this horror for which we are trying to account. We have to ask, what are the conditions in human life and mind that have brought about the widespread aversion to incest that is so generally manifested. According to Westermarck, these conditions are to be found in the process of natural selection; marriages between near kin are, he maintains, on the whole injurious to the species and, therefore, through survival of the fittest, the existing races of men show a marked aversion to such marriages. [Sidenote: These effects by no means certain] In estimating the correctness of this theory, it is well to remember that the supposed ill effects of inbreeding in men and animals are by no means as yet universally admitted by those who have studied the subject, and that, even so far as their existence is admitted, they are not yet fully understood or accurately measured. It is indeed often a matter of considerable difficulty to discover any ill effects that may be due to this cause, especially in the case of slow breeding animals, such as Man, and the conclusions that have been arrived at with regard to the human race have to a great extent been derived by analogy from observations made upon lower animals. It would seem to be fairly generally agreed that such ill effects as may exist arise for the most part from the reinforcement or accentuation of hereditary weaknesses and defects that is liable to occur in inbreeding and that members of a perfectly healthy family might continue to mate with one another for several (perhaps for many) generations without evil consequences, though possibly a loss of vigour, strength or fertility might ultimately occur. In the case of the Ptolemies of Egypt and the Incas of Peru inbreeding of this kind has (as we have already observed in another connection) actually been practised and does not seem to have produced any conspicuously bad results. There is some evidence, too, that seems to point to the fact that the supposed ill effects of inbreeding are due to the results of continuously similar environment and conditions of life rather than to the physiological resemblance of the parents, or at any rate that any evil effects of the latter cause may be counterbalanced by a change of abode or of the mode of living[251].