A Hit-or-Miss Universe 



327 



that there was no room left for the 

 activities of a guiding power. Like- 

 wise, in the realm of instincts, when it 

 can be shown that these instincts may 

 be reduced to elementary physico- 

 chemical laws the assumption of design 

 becomes superfluous. 



"If we look at the instincts purely as 

 observers we might well get the impres- 

 sion that they cannot be explained in 

 mechanistic terms. We need only con- 

 sider what mysticism apparently sur- 

 rounds all those instincts by which the 

 two sexes are brought together and by 

 which the entrance of the spermatozoon 

 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 physico- 

 chemical explanation ; for example, the 

 curious experiment of Steinach on the 

 reversal of the sexual instincts of the 

 male [rat] whose testes had been 

 exchanged for ovaries. There is little 

 doubt that in this case the sexual ac- 

 tivities of each sex are determined by 

 specific substances formed in the in- 

 terstitial tissue of the ovary and testis. 

 The chemical isolation of the active 

 substances and an investigation of 

 their action upon the various parts of 

 the body would seem to promise fur- 

 ther progress along this line. 



"Marchal's observations on the lay- 

 ing 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 larvae 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 hap- 

 pens is that these workers, when there 

 are no larvae, can consume the food 

 which would otherwise have been de- 

 voured by the larvae; and some sub- 

 stance contained in this food induces 

 the development of eggs in the other- 

 wise dormant ovaries." What ap- 



peared at first sight as a mysterious 

 social instinct is revealed as a simple 

 and commonplace effect of nutrition. 



REACTIONS TO LIGHT 



" If we wish to show in an unmistake- 

 able 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 is the reaction 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 animals, from the 

 lowest forms up to the fishes, are at 

 certain stages — ^very often the larval 

 stage — of their existence slaves to the 

 light." 



Numerous experiments are cited 

 which do indeed prove conclusively 

 that many animals react to light in a 

 perfectly definite way. A tendency 

 to move when stimulated by light is 

 called heliotropism. "A most interest- 

 ing example of the role of heliotropism 

 in the preservation of a species is 

 shown in the caterpillars of Porthesia 

 chrysorrhoea. The butterfly lays its 

 eggs upon a shrub. The larvae hatch 

 late in the fall and hibernate in a nest 

 on the shrub, as a rule not far from the 

 ground. As soon as the temperature 

 reaches a certain height, they leave the 

 nest; under natural conditions, this 

 happens in the spring when the first 

 leaves have begun to form on the shrub. 

 (The larvae can, however, be induced 

 to leave the nest at any time in the 

 winter, provided the temperature is 

 raised sufficiently.) After leaving the 

 nest they crawl directly upward on the 

 shrub where they find the leaves on 

 which they feed. Should the cater- 

 pillars move down the shrub, they 

 would starve to death, but this they 

 never do. What gives the caterpillar 

 this never-failing certainty which saves 

 its life, and for which a hmnan being 

 might envy the little larva? Is it a 

 dim recollection of experiences of former 

 generations? It can be shown that 

 it is the light reflected from the sky 

 which guides the animal upward. When 

 we put these animals into a horizontal 



