1908-9. ] HuMAN EVOLUTION AND HUMAN DISEASE. 539 
Change of food is probably another condition which materially 
affected the developing human race’s tendency to disease. When and 
how this change took place we cannot say; our present day anthropoids 
in their natural state feed almost entirely upon fruit and nuts, but all 
observers say that in their wild condition they may devour young birds 
and possibly other animal life, if they obtain it. In captivity they readily 
take to a carnivorous diet. The change probably took place more or less 
suddenly and certainly all our earliest human records show primitive 
man to be definitely carnivorous or omnivorous in his habits. Whenever 
it became general it must have greatly altered human metabolism and it 
probably actually led to the introduction of certain diseases; it would also 
alter the normal resistance of the body to introduced or existing diseases. 
The definite adoption of the upright habit must also have had an 
exceedingly important influence on human disease. The step from 
the quadrupedal to the bipedal carriage was such a tremendous change 
for the human animal that all sorts of consequences flowed from it, as 
Metschnikoff calls them dysharmonies, which influence even at the present 
time our health and our development. 
But most important of all was a fact which was first pointed out by 
Darwin, and which recently von Hansemann has discussed in a most inter- 
esting manner at the Lisbon International Congress of Medicine, that is 
the fact that the greatest step in human evolution was the ability of the 
human race to rise superior to the ordinary laws of natural selection. 
Among wild animals natural selection rigidly separated the weak and 
physically unfit, but in even the earliest human community there was a 
protection against certain of these selective influences. Its power was 
slight at first but it was nevertheless far-reaching. 
The first human forms which were wise enough to take to a cave 
dwelling necessarily escaped the selecting influences of inclement weather 
which might prove fatal to their more exposed, free living, related forms. 
But such selection among the wild forms serves to destroy the weaker 
individuals; it selects the strong. The cave dwelling brought about a 
certain equality between one physically weak and the physically strong 
and thus one of the most important influences in the propagation of purely 
human disease developed. In the human family to-day, in contra-distine- 
tion to wild animals, a very striking phenomenon of infective disease is 
the numerous examples of subacute or chronic types. Many of our most 
serious infections, even those which are not normally looked upon as 
chronic, are frequently propagated by the persistence of it in a more or 
less chronic form in certain individuals. This is very true of tuberculosis, 
