286 Habit and Instinct. 



regarding, as seems not unreasonable, youthful plasticity 

 as a mediating term between the two extremes of stability, 

 congenital and acquired, there are, as current biological 

 literature shows, two opposing views as to the relations 

 which stability, acquired and congenital, bear to each other. 

 According to the one view they are directly connected 

 through heredity. The acquired stability of one generation 

 becomes through inheritance the basis of the congenital 

 stability of the rest. The adaptation rendered possible by 

 youthful plasticity affords the basis of evolutionary progress, 

 while the inheritance of the acquired stability gives the 

 element of continuity in evolution. According to the 

 opposing view, on the other hand, the two types of stability, 

 congenital and acquired, are completely independent. 

 Individual adaptation through youthful plasticity and 

 habitual repetition, plays no part whatever in racial 

 progress, and contributes nothing to congenital stability. 



Both sides may invoke the aid of natural selection, 

 but in very different degree. To the transmissionist it is 

 merely an accessory aid, of value in keeping up a general 

 standard of organic efficiency; to the selectionist it is 

 vital to the very existence of specialized types of organic 

 stability. 



Let us translate this from the abstract into concrete 

 terms. It is the habit of the house martin to build beneath 

 the eaves. Forsaking its ancestral rocky haunts, it has 

 been led to accommodate itself to and utilize the houses 

 that man has built. This, we may fairly assume, was 

 originally due to individual plasticity, the adaptation to 

 new circumstances being the outcome of intelligence. But 

 it has long since ceased to be an individual matter. It 

 is characteristic of all house martins now ; hence the 

 name. That which was an acquired habit in certain house 

 martins of the past has become, it is argued, through 



