198 ORIGIN OF SPECIES 



been modified through this comparatively sudden mode of 

 transition, I can form no opinion ; but if this has occurred, it 

 is probable that the differences between the young and the 

 mature, and between the mature and the old, were primor- 

 dially acquired by graduated steps. 



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 

 transitional gradations, yet undoubtedly serious cases of dif- 

 ficulty 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 sur- 

 prising, 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 elec- 

 trical, 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 dis- 

 charge; and, as Dr. Radcliffe insists, "in the electrical ap- 

 paratus 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, in- 

 stead of being peculiar, may be only another form of the dis- 

 charge which attends upon the action of muscle and 



