302 PHYSIOLOGICAL DEVELOPMENT. 



the excreted substance is moulded, the protruding growth may 

 be rendered larger and more involved, as we see it in quills 

 and feathers. So that insensible steps, thus indicated in 

 principle, carry us from the exfoliation of epidermis by a fiat 

 surface, to the exfoliation of it by a hollow simple sac, an 

 introverted sac, and a sac further complicated ; each of which 

 produces its modified kind of tegumentary appendage. 



§ 295. Among many other differentiations of the outer 

 tissues, the most worthy to be noticed in the space that re- 

 mains, are those by which organs of sense are formed. We 

 will begin with the simplest and most closely allied to the 

 foregoing. 



Every hair that is not too long or flexible to convey to its 

 rooted end a strain put upon its free end, is a rudimentary 

 tactual organ ; as may be readily proved by touching one of 

 those growing on the back of the hand. If, then, a creature 

 has certain hairs so placed that they are habitually touched 

 by the objects with which it deals, or amid which it moves, 

 an advantage is likely to accrue if these hairs are modified 

 in a way that enables them the better to transmit the im- 

 pressions derived. Such modified hairs we have in the 

 vibrissa?, or, as they are commonly called, the " whiskers " 

 possessed by Cats and feline animals generally, as well as by 

 Seals and many Rodents. These hairs are long enough to 

 reach objects at considerable distances ; they are so stiff that 

 forces applied to their free ends, cause movements of their 

 imbedded ends ; and the sacs containing their imbedded ends 

 being well covered with nerve-fibres, these developed hairs 

 serve as instruments of exploration. By constant use of them 

 the animal learns to judge of the relative positions of objects 

 past which, or towards which, it is moving. When stealthily 

 approaching prey or stealthily escaping enemies, such aids to 

 perception are obviously important : indeed their importance 

 has been proved by the diminished power of self-guidance in 

 the dark, that results from cutting them off. These, then, are 



