nS DIFFICULTIES OF TEE THBOUT 



the previous and earlier stages of development would in 

 some cases be hurried through and finally lost. Whether 

 species have often or ever been modified through this com- 

 paratively 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 be- 

 tween the mature and the old, were primordially 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 difficulty occur. 



One of the most series is that of neuter insects, which 

 are often differently constructed from either the males or 

 fertile females; but tliis 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 defense, and perhaps for secur- 

 ing prey; yet in the ra,y, 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. More- 

 over, in the ray, besides the organ just referred to, there 

 is, as Dr. E. McDonnell 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 tor- 

 pedo. It is generally admitted that there exists between 

 these organs and ordinary muscle a close analogy, in inti- 

 mate 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 con- 

 traction is accompanied by an electrical discharge; and, as 

 Dr. Eadcliffe 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 



