ON HEREDITY. 91 



organs which are now more frequently used, is dependent upon 

 hereditary influences alone. 



It is quite possible that it depends, on the one hand, upon the 

 suspension of natural selection, or panmixia (and these 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 hereditary 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 selec- 

 tion. 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 improve- 

 ment 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 extermination of those 

 which are unwary ? It may be urged in opposition to this explana- 

 tion 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 



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



