1908-9.] Human EVOLUTION AND HuMAN DISEASE. 543 
in man which favours the development of apical pulmonary tuberculosis 
and, as a result, general pulmonary tuberculosis with the increased inct- 
dence of infection among the immediate companions of the infected person. 
Thus in tuberculosis all the factors which I have outlined have played 
their part in converting a disease of comparatively little importance 
among wild mammals to one of enormous importance among men with 
in addition the result upon the bacillus that it has taken on new biological 
characters which now distinguish it from the form from which it originated. 
Tuberculosis is perhaps the best example I can give, but we have 
another humandisease—typhoid fever, due to a parasitic micro-organism, 
which probably illustrates the manner in which human infectious disease 
has evolved. The bacillus of typhoid fever is an organism which, when 
cultivated in a pure state, shows characters which we have been accustomed 
to regard as specific. These characters are not always constant and we 
find that the more we study races of typhoid bacilli, isolated from different 
cases and epidemics, that differences become apparent, some of them slight, 
some of them marked, and we are forced to conclude that spontaneous 
variations occur. More than this, however, when we extend our survey 
to bacteria-producing diseases in other animals, such as the bacillus typhi- 
murium, a related organism of rats, the hog cholera bacillus, the organism 
found in epidemics of meat poisoning and several other forms, we see how 
striking are the resemblances to the human bacillus in all their biological 
characters but especially, as has been pointed out, in a recent paper by 
Zopnik in the peculiar symptoms of the diseases they produce in the 
different animals they infect, the ulceration of the lymphatic apparatus 
of the intestine, the lesions which are found in the lymphatic glands, 
spleen and bone marrow, even the curious diminution of leucocytes in the 
blood stream. All these bacteria aredistinct species and man is only infected 
by the varieties of typhoid bacilli, and the so-called paratyphoid forms, never 
with bacillus typhi murium or hog cholera, but one cannot help think- 
ing that the resemblances point to a common origin and when we pursue 
the enquiry still farther we find that the nearest form which is not ordi- 
narily disease-producing is the bacillus coli communis, which inhabits 
the intestine. It is possible that epidemics of a typhoid-like disease 
existed among matmmalia prior to man’s advent, but human domestication 
early gave rise to conditions which would favour the evolution of these 
diseases. ‘he sanitary conditions of the cave man must have been 
pretty favourable for the contamination of food and water with filth, 
and conditions for the rapid spread of enteric diseases would be everywhere 
present. We do not need even to assume that our present day typhoid 
is to be traced to a prehuman disease. ‘There are to-day, and have been 
to a much more marked degree among uncivilized man, conditions which 
