CH. xii.] ADJUSTMENT, DIRECT AND INDIRECT. 61 



In all these cases, here too hriefly summed up, natural 

 Beleetion must of course be regarded as steadily cooperating 

 with direct adaptation. No matter whether individual vari- 

 ations are directly called forth by environing agencies, or are 

 due to internal causes, in our ignorance of which we call 

 them fortuitous, they must equally be the objects of natural 

 selection wherever they influence, in the slightest degree, the 

 individual's chances of survival. Thus the theory of natural 

 selection is not superseded, but supplemented, by the class 

 of considerations here suggested by Mr. IMivart's objections. 

 Ordinarily, if not always, the two processes must go on in 

 concert ; and while the frequent occurrence of directly adap- 

 tive changes must greatly accelerate the operation of natural 

 selection, on the other hand natural selection, by weeding out 

 all cases of retrograde variation, must complete the work of 

 direct adaptation. 



There are, however, some conspicuous instances in which 

 natural selection seems to play either a very subordinate 

 part, or none at all. As we have just been considering eyes 

 and ears, let us once more return to them, to show how certain 

 peculiarities in their structure must be chiefly due to directly 

 adaptive changes. Within the human ear, firmly fastened 

 in the temporal bone, is a spirally-coiled chamber, known as 

 the cochlea. Within this chamber there is a very elastic 

 membrane, and on it lie the so-called Jihres of Corti, which 

 are a series of fibrous filaments placed side by side, with 

 great regularity, so as to present somewhat the appearance of 

 the key-board on a piano. It is now held by physiologists 

 shat this row of fibres is really a key-board, and that each 

 fibre is set in vibration only by a particular musical note, 

 exactly as an A-tuning-fork is set vibrating when A ia 

 sounded near it, but not when any other note is sounded. 

 The auditory nerve, in passing into the cochlea, branches into 

 an immense number of nerve-filaments, each of which com- 

 municates with one of the ke^s of this ear piano. So that 



