92 ON HEREDITY. [II. 



effects would be transmitted), and on the other hand upon the 

 direct influence of increased use during the course of a single 

 life. We do not yet know with any accuracy, the amount of 

 change which may be produced by increased use in the course 

 of a single life. If it is desired to prove that use and disuse 

 produce hcreditar^^ effects without the assistance of natural 

 selection, it will be necessary to domesticate wild animals (for 

 example the wild duck) and preserve all their descendants, 

 thus excluding the operation of natural selection. If then all 

 individuals of the second, third, fourth and later generations of 

 these tame ducks possess identical variations, which increase 

 from generation to generation, and if the nature of these 

 changes proves that they must have been due to the effect of 

 use or disuse, then perhaps the transmission of such effects 

 may be admitted ; but it must always be remembered that 

 domestication itself influences the organism, — not only directly, 

 but also indirectly, by the increase of variability as a result of 

 the suspension of natural selection. Such experiments have 

 not yet been carried out in sufficient detail \ 



It is usually considered that the origin and variation of 

 instincts are also dependent upon the exercise of certain 

 groups of muscles and nerves during a single life-time ; and 

 that the gradual improvement which is thus caused by practice, 

 is accumulated by hereditary transmission. I believe that this 

 is an entirely erroneous view, and I hold that all instinct is 

 entirely due to the operation of natural selection, and has its 

 foundation, not upon inherited experiences, but upon the 

 variations of the germ. 



Why, for instance, should not the instinct to fly from enemies 

 have arisen by the survival of those individuals which are 

 naturally timid and easily startled, together with the extermina- 

 tion of those which are unwary? It maybe urged in opposition 

 to this explanation that the birds of uninhabited islands which 

 are not at first shy of man, acquire in a few generations an 

 instinctive dread of him, an instinct which cannot have arisen 

 in so short a time by means of natural selection. But in this 

 case are we really dealing with the origin of a new instinct, 

 or only with the addition of one new perception ('Wahrneh- 



' C. Darwin, 'Variation of Animals and Plants under Domestication,' 

 vol. i. 



