d-mouth disease_--are
communicable to two species; while the remainder are almost absolutely
confined to one species, even though this be thrown into closest contact
with half a dozen others.

Again, we have half a dozen similar instances in the case of
tuberculosis itself. The horse and the sheep, for instance, are both
most intimately associated with cattle, pastured in the same fields, fed
upon the same food, and yet tuberculosis is almost unknown in sheep and
decidedly uncommon in horses, and when it does occur in them is from a
human source. The goat is almost equally immune from both human and
_bovine_ forms, while the cat and the dog, although developing the
infection with a low degree of frequency, almost invariably trace that
infection to a human source.

There is, therefore, no _a priori_ reason whatever why we should be any
more susceptible to bovine tuberculosis than the remainder of the
domestic animals. It is only fair to say, however, that the animal whose
diet--and appetite--most closely resembles ours, the hog, is quite
fairly susceptible to bovine tuberculosis if fed upon the milk or meat
of tuberculous cattle.

Next came the particularly consoling fact that although nothing has been
more striking than the great increase in the amounts of meat and milk
consumed by the mass of the community during our last twenty years'
progress in civilization, this has been accompanied not by any increase
of tuberculosis, but by a _diminution of from thirty-five to forty-five
per cent_. The allegation so frequently made that there has been an
increase in the amount of infantile tuberculosis has been shown, upon
careful investigation by Shennan of Edinburgh, Guthrie of London, Kossel
in Germany, Comby in France, Bovaird in New York, and others, to be
practically without foundation.

Then, while repetitions of Koch's experiment, upon which his
announcement was based, of inoculating calves and young cattle with
_human_ bacilli have proved that a certain number of them can be, under
appropriate circumstances, made to develop tuberculosis, that number has
never been a large percentage of the animals tested, and in many cases
the infection has been a local one, or of a mild type, which has
resulted in recovery. Lastly, while a number of bacilli, with _bovine_
culture and other characteristics, have been recovered from the bodies
of children dying of tuberculosis, and these bacilli have proved
virulent to calves when in