THE INSTINCTS OF ANIMALS 159 



Seitz relates of the largest of all these spinners, the Chinese Attacus 

 atlas, that this silk sheath ' is continued to the nearest strong branch, 

 so that it is impossible with the hand to detach the leaves that con- 

 ceal an Atlas-pupa from the tree.' To be sure, this pupa weighs 

 about eleven grammes ! 



Since instincts vary, as well as the visible parts of an animal, 

 a fulcrum is afforded by means of which selection can bring about all 

 these very special adaptations to given conditions, since it always 

 preserves for breeding the best suited variations of an already 

 existing instinct. Any other interpretation is once more excluded. 



The same may be said of insects and their egg-laying. This, too, 

 is in many cases only performed once in a lifetime, and the insect 

 dies before it has seen the fruit of its labour. Yet egg-laying is 

 performed in the most effective manner, and with the most perfect 

 security of result. It seems as if the insect knew, so to speak, 

 exactly where, in what numbers, and how it should lay its eggs. 

 Many Mayflies (Ephemeridas) let their eggs fall all at once into the 

 water in which the larvae live ; many Lepidoptera, such as Macvoglossa 

 stellatarum, lay their eggs singly, and on definite plants — the humming- 

 bird hawk-moth, just referred to, on Galium mollugo; others, like 

 Melitcea cirtxia, lay their eggs in heaps on the leaves of the way-bread 

 (Plantago media), or, like Aglia tau, on the bark of a large beech- 

 tree. Nothing in these different modes of egg-laying is due to chance 

 or caprice ; all is determined and regulated by instinct, and all, as far 

 as we can see, is as well adapted to its purpose as possible. When, 

 for instance, Macroglossa stellatarum lays her eggs singly, or in twos 

 or threes, on the green leaves of the food-plant, it thereby obviates 

 the danger of scarcity of food for the comparatively large caterpillars, 

 since not many of them could subsist together on a single plant of 

 Galium, while Aglia tau can place several hundred eggs on the same 

 beech-tree trunk without having to fear that its caterpillars will not 

 find abundant nourishment. The precision with which the egg-laying 

 instinct works is even greater in other species in which there are 

 more special requirements, e. g. when the eggs have to be laid on the 

 under side of the leaves, as in Vanessa prorsa, or where they have to 

 be cemented together in a little pillar, so that they bear a deceptive 

 resemblance to the green flower-buds of the food-plant (the stinging- 

 nettle). 



It is certainly astonishing how exactly the stimulus in these 

 cases is specialized to the liberation of the instinct. In general the 

 smell of the food-plant of the caterpillar is enough for most butter- 

 flies, and this attracts the female ready to deposit its eggs, but 



