538 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vou. VIII. 
condition is more or less true of all wild animals, but not universally so, 
as we sometimes hear of epidemics sweeping through whole areas, mate- 
rially reducing the number of given species found in a district, such for 
instance as the epidemics which periodically sweep off thousands of 
rabbits in the northwest. But, even with these exceptions, conditions 
are not favourable among wild forms for the transmission of epidemics, 
With the evolution of the earliest human forms there was probably more 
or less breaking down of this isolation between families. The earliest hu- 
man families must have been compelled, for the sake of mutual protection, 
to associate more or less closely with one another and out of the family 
group developed the tribal group with its closer tribal life and the resulting 
greater opportunities for the transmission within the tribe of acquired 
infections and parasites. But the change in domestic habits must have 
played an enormous part in favouring the spread of parasitic diseases. 
The anthropoids are all arboreal forms. ‘Their life is spent almost entirely 
in the trees and, in the forests, they move about over a wide area, living 
continuously in the open air. Apparently several of these higher apes 
build some sort of nest or sleeping shelter in the trees, but all writers 
agree, that these are quite temporary structures and are never used for 
many days in succession. They are of slight importance for the trans- 
mission of parasites. How important this nest life is for the passage of 
disease from one animal to another is seen in the birds where the nests 
become infected with parasitic insects which apparently serve for the 
transmission from the parents to the nestling of the blood diseases which 
are so common among birds. Some time, however, the anthropoid for- 
sook the trees and took to living on the surface of the ground. One 
writer suggests, that this important step in human evolution was forced 
upon the anthropoid population by an enormous forest fire, which denuced 
thousands of square miles of country of the forest growth which sheltered 
them. ‘This does not perhaps seem likely since forest fires do not occur 
in the tropical regions where the anthropoids live and where man most 
probably had his origin, but are peculiar to temperate zones. However, 
when the parent form took to living on the ground they were naturally 
driven to cave dwellings for shelter. This becomes more marked the 
more they migrated to temperate climates and required the shelter during 
the inclement seasons of the year. But it probably was forced upon the 
race by the onset of a glacial period. Caves, however, are not common 
and the cave man, when he secured one, retained it against all comers; 
it became his castle and was the home of himself, his family and descend- 
ants, until it became too small to accommodate them. Here then we 
have developed a close and intimate association which favoured 
the spread of infections, once acquired. 
