1908-9. | HuMAN EvoLuTION AND HUMAN DISEASE. 537 
cestors and, in the second place, because we know so little of the diseases 
of our nearest animal relations, the anthropoids. But there is no doubt 
that many of the purely human infections must have begun fairly low 
down in the scale of human evolution since we find that the 
nearer the ape approaches the human form the more certain it is to be 
infected by purely human parasites. This is seen for instance in the 
human disease, syphilis, in which Metschnikoff and, more recently, Neisser 
have shown that the chimpanzee and orang can be infected, developing 
the disease almost as typically as a man whilst lower monkeys like the 
macacus only acquire an incomplete infection. There are many other 
examples such as this, which demonstrate, that if we wish to study human 
infectiousdisease in another animal, the most promising results are obtained 
from a study of the anthropoids. But we must not think from this that 
these diseases have originated with the anthropoids. ‘This is most unlikely 
and in many cases, as far as we know, these purely human diseases do not 
occur among them. It is because of the similarity of tissue structure 
and blood conditions that the human parasite finds conditions favourable 
for development, when introduced into the higher apes. In no cases do 
these animals acquire these purely human diseases with the same cer- 
tainty as the lowest human individual would acquire them. Infection 
when it occurs is an artificial condition, not one obtaining in nature. 
I think we must admit, therefore, that the human diseases we are consider- 
ing must have originated when the race became more or less like the race 
of the present time, whether at the stage of Dubois’ Pithecanthropos 
or in the later stages of the cave man, and it seems probable that there 
were, with the evolution of early man, new conditions set up which fa- 
voured the evolution of purely human disease. Let us consider fora mo- 
ment what these conditions probably were. The best known anthropoids 
are more or less solitary animals. The gorilla, according to von Koppen- 
fels and other writers, lives in single family groups, consisting of the 
male and female and their offspring of various ages. Apparently as soon 
as the young males reach maturity a struggle arises between them and 
the old male and to the victor belong the females of the group; the 
vanquished being either killed or driven out. The chimpanzee apparently 
lives in small families or family groups. The orang also seems to live in 
small families, consisting of male and female and half-grown young. 
According to some writers, the adult male orangs live alone, except during 
the pairing season. In all these animals there seems to be a consider- 
able separation of the family groups and the separation is maintained 
largely on account of the sexual jealousy of the males. 
There is among these wild progenitors of man an isolation of individual 
groups which prevented the transmission of epidemic diseases. This 
