150 DIFFICULTIES OF THE THEORY '_Ohap. VI. 



Special Difficulties of the Theory of Natural Selection. 



Although we must be extremely cautious in concluding that any 

 organ could not have been produced by successive, small, tran- 

 sitional gradations, yet undoubtedly serious cases of difficulty occur. 



One of the most serious is that of neuter insects, which are often 

 differently constructed from either the males or fertile females ; but 

 this case will be treated of in the next chapter. The electric 

 organs of fishes offer another case of special difficulty; for it is 

 impossible to conceive by what steps these wondrous organs have 

 been produced. But this is not surprising, for we do not even 

 know of what use they are. In the Gymnotus and Torpedo they 

 no doubt serve as powerful means of defence, and perhaps for 

 securing prey; yet in the Ray, as observed by Matteucci, an 

 analogous organ in the tail manifests but little electricity, even 

 when the animal is greatly irritated; so little, that it can 

 hardly be of any use for the above purposes. Moreover, in the 

 Ray, besides the organ just referred to, there is, as Dr. R. M'Donnell 

 has shown, another organ near the head, not known to be electrical, 

 but which appears to be the real homologue of the electric battery 

 in the Torpedo. It is generally admitted that there exists between 

 these organs and ordinary muscle a close analogy, in intimate 

 structure, in the distribution of the nerves, and in the manner in 

 which they are acted on by various reagents. It should, also, be 

 especially observed that muscular contraction is accompanied by an 

 electrical discharge ; and, as Dr. Radcliffe insists, " in the electrical 

 apparatus of the torpedo during rest, there would seem to be a 

 charge in every respect like that which is met with in muscle and 

 nerve during rest, and the discharge of the torpedo, instead of being 

 peculiar, may be only another form of the discharge which attends 

 upon the action of muscle and motor nerve." Beyond this we can- 

 not at present go in the way of explanation ; but as we know so 

 little about the uses of these organs, and as we know nothing about 

 the habits and structure of the progenitors of the existing electric 

 fishes, it would be extremely bold to maintain that no serviceable 

 transitions are possible by which these organs might have been 

 gradually developed. 



These organs appear at first to offer another and far more sericus 

 difficulty ; for they occur in about a dozen kinds of fish, of which 

 several are widely remote in their affinities. When the same organ 

 is foucd in several members of the same class, especially if in 

 mcmbeis having very different habits of life, we may generally 

 attribute its presence to inheritance from a common ancestor ; and 



