INSTINCT 237 



possibly affect the males and fertile females, which alone leave 

 descendants. I am surprised that no one has hitherto advanced this 

 demonstrative case of neuter insects, against the well-known doc- 

 trine of inherited habit, as advanced by Lamarck. 



SUMMARY 



I have endeavored in this chapter briefly to show that the men- 

 tal qualities of our domestic animals vary, and that the variations 

 are inherited. Still more briefly I have attempted to show that in- 

 stincts vary slightly in a state of nature. No one will dispute that 

 instincts are of the highest importance to each animal. Therefore, 

 there is no real difficulty, under changing conditions of life, in 

 natural selection accumulating to any extent slight modifications 

 of instinct which are in any way useful. In many cases habit or 

 use and disuse have probably come into play. I do not pretend that 

 the facts given in this chapter strengthen in any great degree my 

 theory; but none of the cases of difficulty, to the best of my judg- 

 ment, annihilate it. On the other hand, the fact that instincts are 

 not always absolutely perfect and are liable to mistakes; that no 

 instinct can be shown to have been produced for the good of other 

 animals, though animals take advantage of the instincts of others ; 

 that the canon in natural history of "Natura non facit saltum," is 

 applicable to instincts as well as to corporeal structure, and is 

 plainly explicable on the foregoing views, but is otherwise inex- 

 plicable — all tend to corroborate the^theory^oXxiaLuxal selection. 



This theory is also strengthened by some few other facts in re- 

 gard to instincts; as by that common case of closely allied, but 

 distinct, species, when inhabiting distant parts of the world and 

 living under considerable different conditions of life, yet often re- 

 taining nearly the same instincts. For instance, we can under- 

 stand, on the principle of inheritance, how it is that the thrush of 

 tropical South America lines its nest with mud, in the same pecu- 

 liar manner as does our British thrush; how it is that the Hornbills 

 of Africa and India have the same extraordinary instinct of plas- 

 tering up and imprisoning the females in a hole in a tree, with only 

 a small hole left in the plaster through which the males feed them 

 and their young when hatched; how it is that the male wrens 

 (Troglodytes) of North America build "cock-nests," to roost in, 

 like the males of our Kitty-wrens — a habit wholly unlike that of 

 any other known bird. Finally, it may not be a logical deduction, 

 but to my imagination it is far more satisfactory, to look at such 

 instincts as the young cuckoo ejecting its foster-brothers, ants 



