Origin of Species Variorum

Comparison with 1866

absence in some of the members to its loss through disuse or natural selection. But if the electric organs had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other. Nor does geology at all lead to the belief that formerly most fishes had electric organs, which most of their modified descendants have lost. But when we look closer to the subject, we find in the several fishes provided with electric organs that these are situated in different parts of the body,—that they differ in construction, as in the arrangement 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 (and this is perhaps the most important of all the differences) being supplied through different nerves from widely different sources. Hence in the several remotely allied fishes furnished with electric organs, these cannot be considered 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 the greater difficulty disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have arisen and been developed in each separate fish. ↑ The luminous organs which occur only in a few insects, belonging to widely different families and orders, but which are situated in different parts of their bodies, offer a difficulty almost exactly parallel with that 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 Asclepias,—genera almost as remote as is possible amongst flowering plants. In all these cases of two species, far removed from each other in the scale of organisation, being furnished with a similar anomalous organ, it should be observed that although the general appearance and function of the organ may be identically the same, yet some fundamental difference between them can always, or almost always, be detected.

Text in this page (from paragraph 4200, sentence 710 to paragraph 4200, sentence 730, word 18) is not present in 1866

absence in some of the members 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 belief that .. most fishes formerly possessed electric organs, which .. their modified descendants have now lost. But when we look at the subject more closely, we find in the several fishes provided with electric organs, that these are situated in different parts of the body,— that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited— and lastly, in being supplied with nerves proceeding from different sources, and this is perhaps the most important of all the differences. Hence in the several .. fishes furnished with electric organs, these cannot be considered 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 the .. difficulty of an organ, apparently the same, arising in several remotely allied species, disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have .. been developed in each separate group of fishes. ↑ The luminous organs which occur .. in a few insects, belonging to widely different families, and .. which are situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly parallel with that of the electric organs. Other similar 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 Asclepias,— genera almost as remote as is possible amongst flowering plants; but here again the parts are not homologous. In all cases of beings, far removed from each other in the scale of organisation, which are furnished with similar and peculiar organs, it will be found that although the general appearance and function of the organs may be the same, yet fundamental differences between them can always be detected. For instance, the eyes of cephalopods or cuttle-fish and of vertebrate animals appear wonderfully alike; and in such widely sundered groups no part of this resemblance can be due to inheritance from a common progenitor. Mr. Mivart has advanced this case as one of special difficulty, but I am unable to see the force of his argument. An organ for vision must be formed of transparent tissue, and must include some sort of lens for