tend to preserve those mother-birds which adopted this device. I give this explanation as the only one which either Mr. Darwin or myself has been able to suggest. It will be observed, however, that it is unsatisfactory, inasmuch as it fails to account for the most peculiar feature of the instinct—I mean the trailing of the apparently wounded wing.
The instinct of migration furnishes another case of special difficulty, but, as I have no space to dwell upon the sundry questions which it presents for solution, I shall now pass on to the last of the special difficulties which most urgently call for consideration. The case to which I refer deserves, I think, to be regarded as the most extraordinary instinct in the world. There is a species of wasp-like insect, called the sphex. This insect lays its eggs in a hole excavated in the ground. It then flies away and finds a spider, which it stings in the main nerve-center of the animal. This has the effect of paralyzing the spider without killing it. The sphex then carries the now motionless spider to its nursery, and buries it with the eggs. When the eggs hatch out the grubs feed on the paralyzed prey, which is then still alive and therefore quite fresh, although it has never been able to move since the time when it was buried. Of course, the difficulty here is to understand how the sphex insect can have acquired so much anatomical and physiological knowledge concerning its prey as the facts imply. We might indeed, suppose, as I in the first instance was led to suppose, that the sting of the sphex and the nerve center of the spider being both organs situated on the median line of their respective possessors, the striking of the nerve-center by the sting might in the first instance have been thus accidentally favored, and so have supplied a basis from which natural selection could work to the perfecting of an instinct always to sting in one particular spot. But more recently the French entomologist, M. Fabre, who first noticed these facts with reference to the stinging of the spider, has observed another species of sphex which preys upon the grasshopper, and, as the nervous system of a grasshopper is more elongated than the nervous system of a spider, the sphex in this case has to sting its prey in three successive nerve-centers in order to induce paralysis. Again, still more recently, M, Fabre has found another species of sphex which preys upon a caterpillar, and in this case the animal has to sting its victim in nine successive nerve-centers. On my consulting Mr. Darwin in reference to these astonishing facts, he wrote me the following letter:
"I have been thinking about Pompilius and its allies. Please take the trouble to read on perforation of the corolla, by bees, page 425, of my 'Cross-Fertilization,' to end of chapter. Bees show so much intelligence in their acts, that it seems not improbable to me that the progenitors of Pompilius originally stung caterpillars and spiders etc., in any part of their bodies, and then observed by their intelli-
