f 72 Animal Life and Intelligence. 



diminution from disuse, the reduction of size in parts or 

 organs may be the result of the absence or cessation of 

 selection. If an organ be subject to selection, the mean 

 size in adult creatures will be that of the selected indi- 

 viduals ; but if selection ceases, it will be the mean of those 

 born. Let us suppose that nine individuals are born, and 

 that the size of some organ varies in these from 1, the 

 most efficient, to 9, the least efficient. The birth-mean will 

 therefore be, as shown on the left-hand side of the follow- 

 ing table, at the level of number "5, four being more 

 efficient, and four less efficient. But if, of these nine, six be 

 eliminated, then the mean of the survivals will be as shown 

 xm the right-hand side of the table : 



i 



2 Survival-mean. 

 3 



Birth-rncan 5 I 



rl> Eliminated individuals. 



The result, then, of the cessation of selection will be to 

 reduce the survival-mean to the birth-mean, and that with- 

 out any necessary effect of disuse. But unless this be 

 accompanied by a tendency to diminution due to economy 

 of growth or some other cause, this cannot produce any 

 well-marked or considerable amount of reduction. I very 

 much question, for example, whether the cessation of 

 selection, even with the co-operation of the principle of 

 economy of growth, will adequately account for the reduc- 

 tion to nearly one-half its original proportion of the brain 

 of the duck. The subject will be more fully discussed, 

 however, in the next chapter. 



There is perhaps some tendency for disused parts to 

 be reduced in size through artificial selection, imbecile 

 small-winged ducks, for example, being more readily kept in 

 domestication. On the other hand, the increase of size in 

 organs may presumably, in certain cases, be enhanced by 

 selection. Pigs, for example, have been selected according 



