196 THE POPULAR SCIENCE MONTHLY. 



influence of the medium is so marked, it must have been all-important 

 at the outset while types were undetermined. 



As with plants so with animals, we find good reason for inferring 

 that while all the specialities of the tegumentary parts must be as- 

 cribed to the natural selection of favourable variations, their most 

 general traits are due to the direct action of surrounding agencies. 

 Here we come upon the border of those changes which are ascribable 

 to use and disuse. But from this class of changes we may fitly ex- 

 clude those in which the parts concerned are wholly or mainly passive. 

 A corn and a blister will conveniently serve to illustrate the way in 

 which certain outer actions produce in the superficial tissues, effects of 

 a purely physical kind effects related neither to the needs of the 

 organism nor to its structural proclivities. They are neither adaptive 

 changes nor changes towards completion of the type. After noting 

 them we may pass to allied, but still more instructive, facts. Contin- 

 uous 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 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 inter- 

 mittent pressure, we have the proofs : first, that the tracts most ex- 

 posed 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 evidence, however, full of meaning as they are, I give 

 simply to prepare the way for evidence of a much more conclusive 

 kind. 



"Where ulceration 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 contended that this make-shift skin results from the inherited 

 proclivity of the type the tendency to complete afresh the structure 

 of the species when injured. "We cannot, however, ignore the imme- 

 diate influence of the medium, on recalling the facts above named, or 

 on remembering the further fact that an inflamed surface of skin, 

 when not sheltered from the air, will throw out a film of coagulable 

 lymph. But that the direct action of the medium is a chief factor we 

 are clearly shown by another case. Accident or disease occasionally 

 causes permanent eversion, or protrusion, of mucous membrane. After 



