140 DIFFICULTIES OF THE THEORY [CHAP. VL 



they grow older. Prof. Cope states that the teeth of certain lizards 

 change much in shape with advancing years; with crustaceans 

 not only many trivial, but some important parts assume a new 

 character, as recorded by Fritz Miilier, after maturity. In all 

 such cases, and many could be given, if the age for reproduction 

 were retarded, the character of the species, at least in its adult 

 state, would be modified ; nor is it improbable that 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 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 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 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 diffi- 

 culty; 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 Eay, 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. 

 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 



