No. I.] THE VERTEBRATE EAR. 221 



Because plainly enough the sense organs in this zone were the 

 most suitable structures for natural selection to seize hold of 

 and develop further, both physiologically and morphologically, for 

 the simple but sufficient reason that any sensory impulses strik- 

 ing this single region, so sharply circumscribed, would be propa- 

 gated into two separated centres of the brain simultaneously, 

 and thereby " ring in " a multiple alarm, and this single zone 

 contained the only territory so provided. Owing to the pecu- 

 liarities of the propagation of wave motion, one part of the head 

 would have been as good for this as another, but it was neces- 

 sary for the organs to be confined to small space. There exist 

 in vertebrates to-day (most fishes and aquatic amphibia) simi- 

 lar arrangements which have been produced by the renewal of 

 the sensory structures over the gap left by the insinking ear, 

 and we thus have an incipient, superficial ear, which may in 

 future, if the animals so provided should ever send up another 

 growing stock capable of reaching the degree of differentiation 

 represented by the descendants of ancient fishes, — the mam- 

 mals of to-day, — I say this surface ear may be carried below 

 and produce a second internal ear of the essential characteristics 

 of the existing one. Animals higher than fishes have lost this 

 potentiality, and are in danger of being compelled to give way 

 to some new tribe of creatures which may sometime spring from 

 the more generalized fishes armed with greater powers for the 

 struggle of life, which may mean simply that they will be pro- 

 vided with more perfect sense organs. 



We must not think of the fish type as a cold and fast, immu- 

 table thing, but as something plastic, in ceaseless agitation and 

 strife toward greater perfection of organization, or in other 

 words these vertebrates are still subject to the necessities of 

 variation and survival of the fittest. 



It is unnecessary to argue at great length to show that two 

 neighboring sense organs receiving their nerve supply from two 

 cranial nerves having distinct sets of central connections would 

 be on this account specially suited to give the animal the great- 

 est possible amount of information about what was going on 

 outside, at the same time permitting the apparatus to occupy 

 the least possible space. The ear Anlage was so placed that it 

 might be housed inside the head, leaving the surface of the 

 body free for sense organs of another degree of specialization. 



