CHAPTER X. 



A FEW REMARKS AS TO THE INFLUENCE OF OTHER CONDITIONS 

 OF EXISTENCE. 



Besides those external conditions of animal life which I have 

 treated of in the foregoing chapters, there are others of which 

 the effects in certain cases may be of much greater consequence ; 

 which may indeed not unfrequently neutralise the effects of 

 apparently more important ones, while they may nevertheless 

 at present escape any close investigation. Such, for instance, 

 are the effects of gravitation or pressure, of electricity, of the 

 aggregate condition of the surrounding medium, and many 

 others. They are often apparently insignificant as compared 

 with temperature, light, nutriment, &c., not because they are of 

 themselves unimportant, but only because we know much less 

 of their normal effects on the life and growth of animals than 

 of the conditions we have hitherto been discussing. Their 

 action almost entirely eludes those methods of research that I 

 have hitherto employed and which alone I acknowledge as the 

 right ones ; consequently in the following brief discussion of 

 these points I find myself wholly thrown back on hypothetical 

 interpretations of such observations as have been incidentally 

 made. Nay, the number of these observations is in itself so 

 small, that in many cases they do not even suflB.ce as a basis for 

 such an hypothesis ; and finally it must be acknowledged that 

 sometimes the interpretation hitherto offered of certain facts 

 has been founded on gross errors which, however, are widely 

 diffused and, as it would seem, almost ineradicable. 



The eflfeets of gravitation and pressure. — The selective 

 influence of gravitation, and its bearings on the organisation of 



