1908-9. ] HuMAN EvoLUTION AND HUMAN DISEASE. 541 
We have thus in a variety of conditions, which are all associated 
with the domestication of the human race, the circumstances which 
probably surrounded the evolution of human disease. If we take as a 
concrete example the infectious disease, which probably is responsible 
for the largest number of deaths among civilized man, viz., tuberculosis, 
we see how at every point domestication has favoured the development 
and spread of the disease. The bacillus of tuberculosis as a disease pro- 
ducing organism, although its special medium of attack is found in man 
and his domesticated animals, is found not only infecting mammalia, 
but also birds and even lower vertebrate groups such as the reptilia and 
fishes. Its career as a parasitic organism possibly began early in the 
history of the vertebrata, but with the evolution of this group it has 
changed its character so that we distinguish to-day at least four varieties 
of the bacillus of tuberculosis, viz., that of cold-blooded animals, that 
of birds, the so-called bovine type and finally the human variety. The 
inter-relationship of these varieties is very obscure and a good deal more 
bacteriological study will have to be done before we can be very positive 
of the manner in which they have originated. We know, however, that 
the bacillus found in the cold-blooded animals is more closely retated to 
the bacillus of avian tuberculosis than to the mammalian varieties. The 
bovine and human type were at one time considered identical, but they 
have now been clearly separated as distinct varieties, with distinct variation 
of virulence towards different members of the mammalian group. The 
bovine type ought clearly to be considered the more primitive because 
we find that it is capable of infecting in smaller doses and more certainly 
a larger number of members of mammalian genera than the human form. 
The human form seems to have been more specialized by its especial 
association with the genus homo and, as a result, to have lost some of its 
virulence for other mammals. We practically know nothing of the extent 
of the occurrence of tuberculosis among the wild mammalia. Cases have 
been recorded but the organisms have not been isolated and properly 
studied. Wild animals in captivity, however, readily develop tubercu- 
losis; this is especially true of the apesandherbivora, but when the organism 
is isolated in these cases in zoological gardens, it is found to belong either 
to the human or the bovine type. This probably means, at least as far 
as the occurrence of the human type is concerned, that the captive animals 
have been infected from human beings with whom they have come in 
contact. When the bovine type is discovered we must assume either that 
they have been infected from their food in captivity, especially from milk, 
or that they have developed in confinement a mammalian tuberculosis which 
had infected them before capture. The latter view is not unlikely, but much 
fuller biological study of suchcultures must be made before we can be certain. 
