The applica­tion of our modern knowledge of heredity to human affairs has been discussed in a very original way by Bateson in his address before the British Associa­tion in Sydney to which the reader may be referred.[216]

CHAPTER X

ANIMAL INSTINCTS AND TROPISMS[217]

1. The idea that the organism as a whole cannot be explained from a physico­chemical viewpoint rests most strongly on the existence of animal instincts and will. Many of the instinctive actions are “purposeful,” i. e., assisting to preserve the individual and the race. This again suggests “design” and a designing “force,” which we do not find in the realm of physics. We must remember, however, that there was a time when the same “purposefulness” was believed to exist in the cosmos where everything seemed to turn literally and metaphorically around the earth, the abode of man. In the latter case, the anthropo- or geocentric view came to an end when it was shown that the motions of the planets were regulated by Newton’s law and that there was no room left for the activities of a guiding power. Likewise, in the realm of instincts when it can be shown that these instincts may be reduced to elementary physico­chemical laws the assump­tion of design becomes superfluous.

If we look at the animal instincts purely as observers we might well get the impression that they cannot be explained in mechanistic terms. We need only consider what mysticism apparently surrounds all those instincts by which the two sexes are brought together and by which the entrance of the spermato­zoön into the egg is secured; or the remarkable instincts which result in providing food and shelter for the young genera­tion.

We have already had occasion to record some cases of instincts which suggest the possibility of physico­chemical explana­tion; for example the curious experi­ment of Steinach on the reversal of the sexual instincts of the male whose testes had been exchanged for ovaries. There is little doubt that in this case the sexual activities of each sex are determined by specific substances formed in the interstitial tissue of the ovary and testes. The chemical isola­tion of the active substances and an investiga­tion of their action upon the various parts of the body would seem to promise further progress along this line.

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.