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 



