Marchal’s observations on the laying of eggs by the naturally sterile worker wasps are a similar case. The fact that such workers lay eggs when the queen is removed or when they are taken away from the larvæ may be considered as a manifestation of one of those wonderful instincts which form the delight of readers of Maeterlinck’s romances from insect life. Imagine the social foresight of the sterile workers who when the occasion demands it “raise” eggs to preserve the stock from extinction! And yet what really happens is that these workers, when there are no larvæ, can consume the food which would otherwise have been devoured by the larvæ; and some substance contained in this food induces the development of eggs in the otherwise dormant ovaries. What appeared at first sight as a mysterious social instinct is revealed as an effect comparable to that of thyroid substance upon the growth of the legs of tadpoles in Gudernatsch’s experiment (Chapter VII). 2. If we wish to show in an unmistakable way the mechanistic character of instincts we must be able to reduce them to laws which are also valid in physics. That instinct, or rather that group of instincts, for which this has been accomplished are the reactions of organisms to light. The reader is familiar with the tendency of many insects to fly into the flame. It can be shown that many species of animals, from the lowest forms up to the fishes, are at certain stages--very often the larval stage--of their existence, slaves of the light. When such animals, _e. g._, the larvæ of the barnacle or certain winged plant lice or the caterpillars of certain butterflies, are put into a trough or test-tube illuminated from one side only, they will rush to the side from which the light comes and will continue to do this whenever the orientation of the trough or test-tube to the light is changed; while they will be held at the window side of the vessel if the light or the position of the vessel remains unchanged. This instinct to get to the source of light is so strong that, _e. g._, the caterpillars of _Porthesia chrysorrhœa_ die of starvation on the window side of the vessel, with plenty of food close behind. This powerful “instinct” is, as we intend to show, in the last analysis, the expression of the Bunsen-Roscoe law of photochemical reactions. A large number of chemical reactions are induced or accelerated by light, and the Bunsen-Roscoe law shows that the chemical effect is in these cases, within certain limits, equal to the product of the intensity into the duration of illumination.