THE OPTIMISM OF PATHOLOGY 303 
bolism out of place, out of time, and out of tune, 
and Nature makes short work of such unhandy 
idiosyncrasies. What then of potato disease and 
salmon disease, of fowl cholera and swine fever, 
and big-bud on our currant bushes and bee-disease 
in our hives? The list may be lengthened out, 
but the answer is probably the same in all cases, 
that these diseases are microbic or parasitic, not 
constitutional, and that they occur in artificial, 
humanly-contrived conditions, not in the economy 
of wild Nature. There are very few examples of 
microbic diseases in natural conditions, one of the 
best known being a bacterial disease in sandhoppers, 
and this may, for all we know, have something to 
do with sewage or the like. It is not denied that 
wild animals are sometimes widely infected with 
microbes so that an epidemic results. We know of 
a sort of diphtheria among ring-doves, and it may 
be that some disease was responsible for the extra- 
ordinarily rapid disappearance of the Passenger 
Pigeon. But what is maintained is that such 
occurrences are rare and evanescent, and that they 
are usually traceable to rapid human interference 
—to introducing new tenants into a region, to killing 
off the natural eliminators of the sickly, to per- 
mitting overcrowding, to an infection of the soil 
and water, and so forth. As to grouse, it seems 
that there is no specific disease in this well-nigh 
sacred bird, but that the removal of natural sifting 
agencies allows of the accumulation of weaklings 
and weaknesses. The contingent of parasites which 
