LECTURE VIII 

 THE INSTINCTS OF ANIMALS 



The robber-wasp — Statement of the problem— Material basis of instincts— Instincts 

 are not 'inherited habits' — Instinct of self-preservation — Fugitive instinct: death- 

 feigning— Masking of crabs— Nutritive instinct— Monophagous caterpillars— Div< 

 modes of acquiring food : May-flies, sea-cucumbers, fishes that snare— 'Aberration ' of 

 instinct— Change of instinct during metamorphosis: Eristalis, Sitaris— Imperfection 

 of adaptation points to origin through natural selection — Instinct and will— Instincts 

 and protective coloration— Leisurely flight of Heliconiidas— Rapid fligbt of Papilionidse 

 -Instincts which act only once in a lifetime— Pupation of butterflies— Pupation of the 

 Longicorns— Pupation of tbe silk-moth— The emperor moth— The cocoons of Atlas— 

 Oviposition of butterflies. 



We have hitherto considered animals with especial regard to the 

 variation and re-adaptation of morphological characters, e.g. modifi- 

 cations of form and colour ; and we have now to ask whether their 

 behaviour also is to be referred as to its origin, in whole or in part, to 

 the principle of selection. All around us we can see that animals 

 know how to use their parts or organs in a purposeful manner : the 

 duckling swims at once upon the water; the chicken which has just 

 been hatched from the egg pecks at the seeds lying on the ground; the 

 butterfly but newly emerged from the pupa, as soon as its wings 

 have dried and hardened, knows how to use them in nio-ht : and the 

 predatory wasp requires no instruction to recognize her victim, a 

 particular caterpillar, a grasshopper, or some other definite insect : 

 she knows how to attack it, to paralyse it by stings, and then hesitates 

 not a moment as to what she has to do next; she drags it to her nest, 

 deposits it in one of the cells already prepared for her future brood, 

 lays a single egg upon it, and roofs the cell carefully over. It is only 

 because all these complex acts are so precisely performed, as precisely 

 as if the wasp knew why she performed them, that the species is able 

 to maintain its existence, for only thus can the rearing of the next 

 generation be secured. Out of the egg there slips a little larva, which 

 at once makes for the paralysed victim, feeds upon it. and grows 

 thereby, then, within the shelter of the closed cell, passes through the 

 pupa stage and is transformed into a perfect wasp. Many species 

 of these predatory wasps do not lay the egg directly beside or upon 

 their prey, but lest its movements should endanger their offspring, 



