CiiAi-. YI. OF NATURAL SELECTION. 185 



Tliesc same organs at first appear to offer anotncr and far 

 more serious difficulty ; for they occur in about a dozen kinds 

 of fisli, of Avliich. several arc Avidely remote in their affinities. 

 Generally when the same organ is found in several niemhers 

 of the same class, especially if in mc^mbers having very differ- 

 ent habits of life, we may attribute its presence to inheritance 

 from a common ancestor; and its absence in some of the mem- 

 bers to loss through disuse or natural selection. So that, if 

 the electric organs had been inherited from some one ancient 

 progenitor, we might have expected that all electric fishes 

 would have been specially related to each other; but this is 

 far from the case. Nor does geology at all lead to the Ijclief 

 that most fishes formerly possessed electric organs, which their 

 modified descendants have now lost. But when we look at 

 the subject more closely, wc find in the several fishes provided 

 with electric organs that these are situated in diiferent parts 

 of the body — that they differ in construction, as in the arrange- 

 ment of the plates, and according to Pacini, in the process or 

 means by which the electricity is excited — and lastly, in the 

 requisite nervous power being supplied through different nerves 

 from widely-diffi;rent sources, and this is perhaps the most im- 

 ])ortant of all the diflerences. Hence in the several remotely- 

 atlied fishes furnished with electric organs, these cannot be 

 ccnisidered as homologous, but only as analogous in function. 

 Consequently there is no reason to suppose that they have 

 been inherited from a common progenitor ; for, had this been 

 the case, they would have closely resembled each other in all 

 respects. Thus tlic greater difficulty disappears, leaving only 

 the lesser yet still great difliculty ; namely, by what graduated 

 steps these organs have arisen and been develojied in each sepa- 

 rate grouji of fishes. 



Tiic luminous organs which occur only in a few insects, 

 belonging to widely-different fiunilies and orders, and v.hich 

 arc situated in different jiarts of the body, offer a difficidty 

 almost exactly jiaralhd witli tiiat of the electric organs. Other 

 cases could be given; for instance, in plants, the very curious 

 contrivance of a mass of pollen-grains, borne on a foot-stalk 

 with an adhesive gland, is apparently the same in Orchis and 

 Asclcpias — genera almost as remote as is possible among 

 flowering ])lants. In all such cases of two species, fiir removed 

 from each other in the scale of organization, being furnislied 

 with similar anomalous organs, it should be observed that, al- 

 though the general appearance and function of the organ may 



