Origin of Species Variorum

be effected in the character of the species. It can also be shown that the production of the two sorts of flowers by the same plant has been effected by finely graduated steps. Again, two distinct organs in the same individual may simultaneously perform the same function, and this is a highly important means of transition: to give one instance,—there are fish with gills or branchiæ that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladders, this latter organ ... being divided by highly vascular partitions, and having a ductus pneumaticus for the supply of air. To give another instance from the vegetable kingdom: plants climb by three distinct means, by spirally twining, by clasping a support with their sensitive tendrils, and by the emission of aërial rootlets; these three means are usually found in distinct groups, but some few plants exhibit two of the means, or even all three, combined in the same individual. ↑ In all such cases one of the two organs for performing the same function might be modified and perfected so as to perform all the work, .. being aided during the progress of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be wholly obliterated.

The illustration of the swimbladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a widely different purpose, namely respiration. The swimbladder has, also, been worked in as an accessory to the auditory organs of certain fish. All physiologists admit that the swimbladder is homologous, or "ideally similar" in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swimbladder