THE OPTIMISM OF PATHOLOGY 307 
In the third place, just as evil is a tax on freedom, 
and instability the penalty of genius, so it seems 
legitimate to say that constitutional disease is the 
seamy side of variability. Disease is often just a 
new departure that has gone a little beyond bounds. 
Sometimes it is a physiological slipping down to a 
lower rung of the evolution ladder. We go back 
to our definition that constitutional disease is a 
type of metabolism which is out of place, out of 
time, and out of tune. It is interesting to notice 
that what would be pathological in one animal may 
have become normalized in another; witness the 
threads from the male stickleback’s kidney with 
which he binds the water-weeds into a nest, or the 
necrosis at the base of the stag’s antlers which leads 
to their being shed. And the supra-salivation 
producing the material out of which the sea-swift 
Collocalia makes the fragile “ edible bird’s-nest,” is 
it not nearly akin to ptyalism? Similarly, what is 
normal at one period of life may be pathological at 
another, as we see in the de-differentiation which 
occurs at certain stages in the change of a tadpole 
into a frog, or of a caterpillar into a butterfly. 
Looking backwards, we see that just as pain is in 
part interpretable as a self-preservative danger 
signal, so constitutional disease, practically absent 
in wild Nature, is a warning to man of the dangers 
of artificiality and foolhardy defiance of the funda- 
mental principles of physiology (“fools afflicted 
because of their transgression,” as the Psalmist put 
it). We see also that many processes labeled disease 
