60 THE FACTORS OP ORGANIC EVOLUTION. 



mainly passive. A corn and a blister will conveniently 

 serve to illustrate the way in which certain outer actions 

 initiate in the superficial tissues, effects of very marked 

 kinds, which are related neither to the needs of the organ 

 ism nor to its normal structure. They are neither adaptive 

 changes nor changes towards completion of the type. 

 After noting them we may pass to allied, but still more 

 instructive, changes. Continuous pressure on any portion of 

 the surface causes absorption, while intermittent pressure 

 causes growth : the one impeding circulation and the 

 passage of plasma from the capillaries into the tissues, and 

 the other aiding both. There are yet further mechanically- 

 produced effects. That the general character of the ribbed 

 skin on the under surfaces of the feet and insides of the 

 hands is directly due to friction and intermittent pressure, 

 we have the proofs : first, that the tracts most exposed to 

 rough usage are the most ribbed ; second, that the insides 

 of hands subject to unusual amounts of rough usage, as 

 those of sailors, are strongly ribbed all over ; and third, that 

 in hands which are very little used, the parts commonly 

 ribbed become quite smooth. These several kinds of evi 

 dence, however, full of meaning as they are, I give simply 

 to prepare the way for evidence of a much more conclu 

 sive kind. 



Where a wide ulcer has eaten away the deep-seated layer 

 out of which the epidermis grows, or where this layer has 

 been destroyed by an extensive burn, the process of healing 

 is very significant. From the subjacent tissues, which in the 

 normal order have no concern with outward growth, there 

 is produced a new skin, or rather a pro-skin; for this 

 substituted outward-growing layer contains no hair-follicles 

 or other specialities of the original one. Nevertheless, it 

 is like the original one in so far that it is a continually 

 renewed protective covering. Doubtless it may be con 

 tended that this make-shift skin results from the inherited 

 proclivity of the type the tendency to complete afresh 



