184 DIFFICULTIES OF THE THEORY Chap. VI. 



the ovigcrous frcna in the one family arc strictly homologous 

 ■with the braiicliiiu of tlic other family; indeed, they graduate 

 into each other. Therefore it need not be doubted that the 

 two little folds of skin, -which originally served as ovigerous 

 frena, but -which, likewise, very slightly aid in the act of res- 

 piration, have been gradually con\erte(l by natural selection 

 into branchije, simply through an increase in their size and the 

 obliteration of their adhesive glands. If all pedunculated cir- 

 rijiedes had become extinct, and they have already suffered far 

 more extinction than have sessile cirripedes, who Avonld ever 

 have imagined that the branchioi in this latter family had 

 originally existed as organs for pre\'cnting the ova from being 

 washed out of the sack ? 



iSj^ecial 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 

 transitional gradations, yet vmdoubtedly serious cases of difli- 

 culty occur, some of which will be discussed in my fiiture work. 



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

 often diiFcrently constructed from either the males or fertile 

 females; but this case will be treated of in the next chapter. 

 Tlie electric organs of fishes offer another case of special diffi- 

 culty ; for it is impossible to conceive by what steps these 

 Avondrous organs have been produced. As Owen has remarked, 

 there is much analogy between them and ordinary muscles, in 

 their manner of action, in the influence on them of the nervous 

 power and of stimulants svich as strychnine, and as some be- 

 lieve in their intimate structure. M^e do not even know of 

 -what use these organs are ; though in the Gjinnotus and Tor- 

 pedo they no doubt serve as powerful means of defence and 

 perhaps for securing prey; 3'et in the Kay an analogous organ 

 in the tail, even when greatly irritated, manifests, as lately 

 observed by Matteuccu, but little electricity — so little that it 

 can hardl}^ be of use for such purposes. ^Moreover, in the Kay, 

 besides the organ just referred to, there is, as Dr. K. ^McDonnell 

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

 trical, but which appears to be the real homologue of the elec- 

 tric batter}' in the torpedo. Lastly, as we know nothing about 

 the lineal progenitors of these fishes, it must be admitted that 

 we are too ignorant to be enabled to aflirm that no transitions 

 arc possible, through which the electric organs might have 

 been developed. 



