"THE CAUSATIOX OF DISEASE. 37 



tin individual. It behoves us, then, to examine into the 

 principle of heredity. 



Let us first take a general glance at the process. Man 

 multiplies by gamogenesis — that is to say, two separate elements, 

 .germ and sperm, enter into union, and there is gradually built 

 np man. The whole series of changes depends', of course, 

 upon the structure of the two elements and the nature of 

 the E.* The case may be briefly put thus : By virtue of 

 <: heredity," the offspring tends towards a structural standard, 

 which is a certain mean between the parental structures. 

 Owing to the sexes, this mean is different for each — the male 

 •offspring tending towards one structural type, and the female 

 ■offspring towards another structural type. 



But although there is such a tendency towards a given type, 

 peculiar to each sex, this type is, as a matter of fact, departed 

 from, otherwise children of the same parents would be exactly 

 alike — the male progeny on the one hand, and the female on 

 the other. There must, then, be at work agencies interfering 

 with " Heredity." These agencies are nothing more nor less than 

 peculiarities of environment. If the E were in all cases identical 

 — aud such an identity is totally impossible — there would be a 

 perfect likeness between the sons and between the daughters, 

 tind the likeness would be exact in every particular, even to 

 the subtlest evolution of thought. (It is necessary to observe, 

 however, that, inasmuch as the parents may acquire several 

 structural peculiarities between the births of their first and last 

 children, the parental structures are not fixed quantities, and 

 this would tend to prevent identity among the offspring even 

 though the E were identical. We may conveniently neglect 

 this fact for the present.) But the E is not the same in any 

 two cases, hence there are deviations from the standard. These 

 deviations are called natural variations. The term is generally 

 applied to such of the offspring as differ markedly from the 

 immediate ancestors, but it is clear that the narrowest varia- 

 tion is quite as much a variation as the widest, and, inasmuch 

 tis all the children of the same parents differ, they are all 

 natural variations from this fixed theoretical mean. It is 



* See the remarks on cause. Chairs I., II., and III., Part I. 



