SECT. IV. XVI. I am aware, however, of the argument that may be used in favour of the vulgar opinion. I may be told, that customs and manners are commonly derived from the genius or disposition, and the genius or disposition from temperament. How else could the constitution of parents be communicated to their children, by means of which communication we see them inherit their infirmities? In the same manner then, may their geniuses and dispositions be communicated. XVII. But this argument fails in many parts of it; first, because by the commixture of the two sexes, which is indispensable in generation, there may result to the children a third temperament, unlike to that of both father and mother. Secondly, because it is not probable, that the seminal matter is in all its parts homogeneous; and to this principle, I think, should chiefly be attributed the notable dissimilarity that we find in some brothers. Thirdly, because many different principles have their influence on the temperament; for example, the accidental disposition of the parents at the time of generation, the various affections of the mother during the formation of the fœtus, the alterations of the atmosphere in this period, childrens aliment in their infancy, and many other things. XVIII. From hence I conclude, the vulgar prognostic, that the short or long lives of children, depend upon the much or little time their parents lived, is to the last degree fallible and void of all probability; because from all the principles we have pointed out, the temperament of the parents with respect to the children, may be vitiated or amended; for we every day see healthy children born of sickly parents, and sickly children born of healthy ones. It is true, that there are some diseases which have the stamp or mark of hereditary ones; but I conclude, this originates, or is derived from a vitiated quality which pervades the whole seminal matter; but this is proper or confined to very few diseases, nor is it so proper or certain with regard to those few, as not to be many times falsified. My father was gouty, but I am not so, neither is any one of my brothers.