Besides, the first condition shatters all our hopes. Would we have the helpful Ammophila? Then we must resign ourselves to accepting the Grey Worm. We move in a vicious circle: to produce good we must invoke the aid of evil. The hostile band brings the friendly troop to our fields; but the second cannot live without the first; and the two show an even balance in numbers. If the Grey Worm abound, the Ammophila finds copious provender for her grubs and her race prospers; if the Grey Worm be rare, the Ammophila’s offspring decrease and disappear. This balance between prosperity and decadence is the immutable law that governs the proportions between devourers and devoured. CHAPTER XX THE MODERN THEORY OF INSTINCT The larvæ of the various Hunting Wasps require their prey to be incapable of movement, so that there may be no resistance on the victim’s part, which would be a source of danger to the fragile egg and, later, to the grub. Moreover, for all its lethargy, it must still be alive; for the grub would refuse to feed on a corpse. The fare provided must be fresh meat and not preserved stuff. I have already laid stress on these two antagonistic conditions, immobility and life, and enlarged on them so fully that I need hardly dwell upon them for a second time. I have shown how the Wasp realizes them by the medium of a paralysis which destroys movement and leaves the organic principle of life intact. With a skill which our most famous vivisectors would envy, the insect drives its poisoned sting into the nerve-centres, the seat of muscular incitation. The operator confines herself to one stroke of the lancet, or else gives two, three or more, according to the structure of the particular nervous system and to the number and grouping of the ganglia. The course of the sting is determined by the exact anatomy of the victim. The particular prey of the Hairy Ammophila is a caterpillar, each of whose nerve-centres, which are distant one from the other and to a certain extent independent in their action, occupies a different segment of the insect. This caterpillar, who is a very lively customer, cannot be stored in the cell, with the Wasp’s egg upon his flank, until he has lost all his power of motion. One movement of his body would crush that egg against the wall of the cell. Now the paralysis of one segment would not mean that the next was also rendered incapable of movement, because of the comparative independence of the seats of innervation. It is necessary, therefore, that all the segments, or at least the most important, be operated on, one after the other, from the first to the last. The course which the Ammophila adopts is that which the most experienced of physiologists would recommend: her sting is transferred from one segment to the next, nine separate times over.