wipe them out.

No doubt the first step in this direction is a thorough understanding
of the creature's life-history. This understanding many able mien
are working for. But there is another line of thought that should
not be forgotten, though it is negative--many animals are immune.
Which are they? Our first business is to list them if we would
learn the why of immunity.

Frogs are among the happy ones. One day early in June I took a
wood-frog in my hand. The mosquitoes swarmed about. In a few seconds
30 were on my hand digging away; 10 were on my forefinger, 8 on my
thumb; between these was the frog, a creature with many resemblances
to man--red blood, a smooth, naked, soft skin, etc.--and yet not a
mosquito attacked it. Scores had bled my hand before one alighted
on the frog, and it leaped off again as though the creature were
red hot. The experiment repeated with another frog gave the same
result. Why? It can hardly be because the frog is cold-blooded,
for many birds also seem, to be immune, and their blood is warmer
than man's.

Next, I took a live frog and rubbed it on my hand over an area
marked out with lead pencil; at first the place was wet, but in a
few seconds dry and rather shiny. I held up my hand till 50 mosquitoes
had alighted on it and begun to bore; of these, 4 alighted on the
froggy place, 3 at once tumbled off in haste, but one, No. 32, did
sting me there. I put my tongue to the frog's back; it was slightly
bitter.

I took a black-gilled fungus from a manure pile to-day, rubbed a