324 THE INFLUENCE OF INANIMATE SUKEOUNDINQS. 



tLeir head bent down for many hours together, the forehead 

 acquires a prominent development of the frontal bones. Darwin 

 has shown, moreover, that the forward lop of one ear of the 

 long-eared rabbit induces a corresponding forward growth of 

 almost every bone of the skull on the same side, so that it is 

 perfectly asymmetrical. Nay, even the gx'owing brain appears 

 to exercise a decided influence by the pressure it exerts on the 

 shape of the surrounding bones. But in all these, and many 

 Other cases which might be enumerated, the only fact ascertained 

 with any certainty is that the growing parts themselves, as well 

 as other organs connected with them, may be modified by pres- 

 sure and by their own weight ; no determined standard for 

 estimating this influence is in any instance fixed, and we learn 

 from them absolutely nothing as to how far this influence may 

 be efiicient in determining the production of the normal types 

 in animals now living. So far as I know, as yet only a few 

 attempts have been made to refer the normal form of the skulls 

 of vertebrate animals to the effects of such constant pressure ; 

 the m.ost important of these are certainly those of Lucae in his 

 researches as to the skulls of mammalia, and those of Gudden in 

 bis investigations as to the growth of the skull of the rabbit. 

 Lucae, however, altogether disregards any experimental treat- 

 ment of the question ; and what we learn from Gudden, valuable 

 as it may be to physicians, physiologists, and anthropologists, 

 is of no present value as afibrding any methodical standard for 

 our inquiry. 



The influence of solid bodies. — The aggregate condition of 

 solid bodies must decidedly have a certain influence on the 

 animals whose life is passed in digging or boring into them. 

 The highly sensitive nose of the mole, for instance, must certainly 

 exhibit a quite difierent structure from that of the prairie-dog, 

 or Dipus, which uses it, as I myself have seen in tame indivi- 

 duals, to beat the earth down firmly in its dwelling. The 

 various and extremely dissimilar structures which occur as 

 organs for burrowing in both vertebrate and invertebrate 

 animals, are so perfect in their adaptation to their function that 

 from the structure of the legs, which are the limbs most com- 

 monly employed for this purpose, it is easy to infer the mode of 



