328 THE INFLUENCE OF INANIMATE SHEKOUNDINGS. 



and some-what extensive influence is exerted upon them by the 

 conditions of their position. Nevertheless, they yield no satis- 

 factory conclusion as to the amount of these mechanical effects, 

 though their existence cannot be disputed. 



And we are still worse off with regard to other influences, 

 which certainly exist, although they remain up to the pre- 

 sent time perfectly unintelligible to us. Thus it can hapdly 

 be doubted that the electric tension of the atmosphere or of 

 terrestrial magnetism must have some effect on animals, and 

 attempts have not been wanting to refer certain phenomena of 

 animal life to these causes. Thus, for instance, at one time the 

 view was put forward that migi'atory birds directed their flight 

 towards the magnetic poles in the Old and New World; an 

 opinion which certainly seems to have been based on the tempt- 

 ing but usually misleading principle of Post hoc, ergo propter hoc. 

 "We may, however, congratulate ourselves that the latest and 

 most thorough work on the migration of birds — that of Palmfen 

 — sets aside this mysterious power of the magnetic poles as 

 purely fabulous. Even with regard to the effects of atmospheric 

 electricity, we may surmise their existence, but cannot grasp 

 it, and remain unable to come to any decision ; that the dis- 

 charge of electricity in the form of lightning can occasion death 

 is all that is certain. But this, of course, is of no great impor- 

 tance, as it never can be a factor in the origination of a new 

 species or the extirpation of an old one ; with regard to such 

 results as these, only the feeble electric tension of the atmosphere 

 need be taken into consideration — and this probably is not even 

 perceptible to most animals— since that alone is capable of 

 exerting any constant influence. Lately, and particularly in 

 France, naturalists have begun to investigate the effects of 

 electric currents on animals and on their development, and have 

 already arrived at very remarkable results. Thus, Onimus 

 found that the ova of frogs developed more rapidly at the nega- 

 tive pole of a constsint current than at the positive pole; 

 Wagner supposes that he has observed an effect of electricity 

 in altering the colours and the form of the wings of butterflies. 

 These and the observations of Pigeon, Chauveau, and others 

 do not, however, at present, allow of our applying them to 



