ORIGIN AND DEVELOPMENT OF INSTINCTS. 191 



as to become accidentally beneficial to the animal, then we 

 are bound to believe that natural selection would fix this 

 habit, or its beneficial variations. And the proof that such 

 a process has taken place is given by the fact of their being 

 many instincts — such as the incubating instinct before 

 alluded to — which cannot conceivably have been developed 

 in any other way. Whether or not this instinct began in 

 habits adapted to the protection of the eggs, it is certain that 

 it cannot have begun with any intelligent reference to hatch- 

 ing them ; and it is no less certain that before the instinct 

 attained its present degree of perfection, it must have passed 

 through many stages of variation, few if any of which can 

 have been due to intelligent purpose on the part of the birds. 

 And further proof is rendered, as I have also previously 

 observed, by the fact that many instincts are displayed by 

 animals too low in the zoological scale to admit of our sup- 

 posing that they can ever have been due to intelligence. To 

 give only one illustration, the larva of the caddice ily lives in 

 water and constructs for itself a tubular case made of various 

 particles glued together. If during its construction this case 

 is found to be getting too heavy — i.e., its specific gravity 

 greater than that of the water — a piece of leaf or straw is 

 selected from the bottom of the stream to be added to the 

 structure ; and conversely, if the latter is found to be getting 

 too light, so as to show a tendency to float, a small stone is 

 morticed in to serve as ballast.* In such a case as this it 

 Seems impossible that an animal so low in the zoological 

 scale can ever have consciously reasoned — even in the most 

 concrete way — that some particles have a higher specific 

 gravity than Others, and that by adding a particle of this or 

 that substance, the specific gravity of the whole structure 

 may be adjusted to that of the water. Yet the actions 

 involved are no less clearly something more than reflex; they 

 an- instinctive, and can only have been evolved by natural 

 selection. Similarly, Professor Duncan suggests, in a lecture 

 before the British Association, 1872, that the instinct of the 

 < Idynerus — which forms a tubular ante-chamber and provision- 

 chamber lilled with stung grubs for the future use of offspring 

 which it never saw— probably arose in tins way. M. Padre 

 observed that Beinbea indica lays an egg in a chamber, 



• A Monographic /.' "i and 8t/noptis of //>>• Trichoptera of the 

 European Fauna, lssi, bj Robert li'Lachlan, ET.R.S. 



