68 Heredity and Environment 



logical ones; indeed function and structure are only two aspects 

 of one and the same thing, namely organization. For all morpho- 

 logical characters there are functional correlatives, for functional 

 characters morphological expressions, and if the one is inherited 

 so is the other. But there are certain characters in which the 

 physiological aspect is more striking than the morphological one. 



Longevity. 'For example, longevity is a physiological character 

 which is undoubtedly dependent upon many causes, but in the 

 case of species which differ greatly in length of life there can be 

 little doubt that we are dealing with an inherited character. The 

 great differences in the length of life of an elephant and a mouse, 

 of a parrot and a pigeon, of a cicada and a squash bug, are as surely 

 the result of inherited causes as are the structural differences be- 

 tween these animals. Within the same species different races 

 or lines show characteristic differences in length of life; in the 

 case of man the average length of life is much greater in some 

 families than in others, and life-insurance companies take account 

 of this fact. Even within the same organism certain organs or 

 cells are short-lived, whereas others are long-lived; some cells 

 and organs live only through the early embryonic period, while 

 others live as long as the general organism. 



Other Functional Characters. Obesity is another physiological 

 characteristic which may be inherited; the members of certain 

 families grow fat in spite of themselves, while members of other 

 families remain thin however well fed they may be. Here also 

 many factors enter into the result, but it seems probable that the 

 differentiating factor is an hereditary one. Baldness affects the 

 male members of certain families when they have reached a given 

 age, while in others neither care, dissipation nor age can rob a 

 man of his bushy top. Haemophilia, or excessive bleeding after an 

 injury, which is due to a deficiency in the clotting power of the 

 blood, is strongly inherited in the male line in certain families. 

 Fecundity and a tendency to bear twins or triplets, left-handed- 

 ness, a peculiar lack of resistance to certain diseases, and many 

 other physiological peculiarities are probably inherited. 



