186 THE TREND OF THE RACE 
their lives and generally before adult age. Hamburger states 
that in Vienna 95 per cent of the children of the poor between 
12 and 13 years of age are infected, and he thinks that practically 
all will be infected before they reach adult life. If it should be 
established that most people become tuberculous at an early age, 
the hypothesis that the parent-offspring correlation for tubercu- 
losis is due simply to opportunities for infection will hardly suffice 
to explain the fact. The generality of early infection is a matter 
to be considered in interpreting the significance of the correlation. 
If almost every one has become infected, and thus has the oppor- 
tunity to develop tuberculosis, and if the existence of the more 
severe forms of the disease is more closely associated with blood 
relationship than it is with the surrounding conditions under 
which tuberculosis is apt to become manifest, the evidence would 
strongly point to the importance of the hereditary factor. The 
problem is a difficult one about which there has been considerable 
controversy, and we shall have to await further insight into the 
subject before the precise réle of heredity can be fully established. 
Should the hereditary factor be a potent one it would indicate 
that natural selection is acting to remove the stocks with a tuber- 
cular diathesis. 
That natural selection tends to eliminate stocks with a pro- 
clivity to other diseases is evident. Several diseases such as 
diabetes, Bright’s disease, Huntington’s chorea and others which 
are known to be transmitted are not infrequent causes of death. 
Dwarfism, ichthyosis, xeroderma, albinism, hereditary cataract, 
and deaf mutism, while not in themselves fatal, may lesson the 
chances for leaving offspring and hence lead to the extinction of 
stocks in which they occur. Hzmophilia which is transmitted as 
a sex linked character would tend inevitably to be eliminated 
by natural selection since it greatly increases the danger from 
any wound that causes the loss of blood. Lossen states that 18 
out of the 37 deaths in the celebrated Mampel family were due 
to this malady. The hereditary forms of insanity not only keep 
their victims from propagating their kind, but they often tend 
to shorten their lives. Brower and Bannister state that in the 
