From these facts, it is readily understandable how important becomes the consideration of the marriage of relatives, such as cousins,[63] who are, of course, individuals of the same family line, whose mating brings together like groups of traits, thus strengthening the existence of these traits, whether desirable or undesirable. Cousin marriages, when the family possess traits of mental ability, may result in children who are geniuses; but cousin marriages, when the family line possesses traits of mental inability, may result disastrously with respect to offspring. Family lines possessing traits of mental weakness should most assuredly join only to family lines possessing traits of strength in those regards. In calculating the inheritability of traits, it is also necessary to consider that certain physical, mental and moral traits flower at the arrival of certain ages only. It is necessary to look along the whole line of a life, as traits may exist at one age and not at another. A boy's beard does not appear until puberty. Likewise, other physical and mental and moral traits sometimes do not manifest themselves until specific ages, according to the type of the family breed. Because a parent dies before the development of the trait does not preclude its transmissibility to his offspring. Huntington's chorea, an extremely undesirable trait, does not develop until middle life, but is transmissible to offspring even though the father dies from some other cause before the period when the disease in his own person would be expected to appear. [Sidenote: Results of Specific Matings] [Sidenote: Andalusian Fowl]