THE CAUSATION OF DISEASE. 59 



year. And what is true of the skin is true also of other 

 organs — notably, of the sexual. These, it is well known, 

 undergo marked changes at certain seasons in many of the 

 lower animals, and, as we have already seen, there is a faint 

 tendency to such a periodicity in man. How far the other 

 organs in man partake of this periodic change, itr is impos- 

 sible to tell ; yet who can say that such hidden rhythmical 

 changes may not j largely help to determine the prevalence of 

 certain disorders at particular seasons of the year, even though 

 it be readily acknowledged that alteration of E at the different 

 seasons plays the largest share in determining such preva- 

 lence ? 



II. Sexual Heredity. — When only one individual is 

 concerned in procreation, as in the case of all asexual 

 organisms, the principle of heredity may be put thus : The 

 offspring tends to pass through the same series of changes, 

 and at the same rate as the parents, the rhythms being 

 in fact like all others. Inasmuch, however, as in the case 

 of man, two individuals take part in the process, the off- 

 spring tends to be neither exactly like the one nor the other, 

 but a certain mean of the two. This mean is interfered with 

 by the structural peculiarities of each sex, for each has pecu- 

 liarities connected with the difference of sexual function. But 

 although these peculiarities chiefly relate to the sexual func- 

 tion, they are not confined to it, as we shall presently see. 



The various sexual differences have been undoubtedly acquired 

 during the course of ages, under the different conditions (E) to 

 which each sex has been exposed. This subject has been care- 

 fully worked out by Darwin, who has shown that a character 

 acquired by one sex at one particular period of life tends, in 

 the offspring, to appear in the same sex and at the same 

 period of life. Illustrations of this fact abound in the animal 

 world. Thus, the antlers of the stag attain their full strength 

 when sexual maturity is reached. At this time many males 

 compete for the female, and success depends upon the strength 

 and prowess of the competitors. The horns are powerful 

 weapons, and thus a stag with strong antlers would have an 

 advantage over another having weaker ones, and would be more 

 likely to succeed in the struggle for the female, and to leave 



