The great mortality amongst the insects of Chontales in 1872 has some bearing on the origin of species, for in times of such great epidemics we may suspect that the gradations that connect extreme forms of the same species may become extinct. Darwin has shown how very slight differences in the colour of the skin and hair are sometimes correlated with great immunity from certain diseases, and from the action of some vegetable poisons, and the attacks of certain parasites.* (* "Descent of Man" volume 1 page 242; and "Animals and Plants under Domestication" volume 2 pages 227-230. I have taken the examples given from the same author.) Any varieties of species of insects that could withstand better than others these great and probably periodical epidemics, would certainly obtain a great advantage over those not so protected; and thus the survival of one form, and the extinction of another, might be brought about. We see two species of the same genus, as in many insects, differing but little from each other, yet quite distinct, and we ask why, if these have descended from one parent form, do not the innumerable gradations that must have connected them exist also? There is but one answer; we are ignorant what characters are of essential value to each species; we do not know why white terriers are more subject than darker-coloured ones to the attacks of the fatal distemper; why yellow-fleshed peaches in America suffer more from diseases than the white-fleshed varieties; why white chickens are most liable to the gapes; or why the caterpillars of silkworms, which produce white cocoons, are not attacked by fungus so much as those that produce yellow cocoons? Yet in all these cases, and many others, it has been shown that immunity from disease is correlated with some slight difference in colour or structure, but as to the cause of that immunity we are entirely ignorant.