Page - 195 - in The Origin of Species

MODES OF TRANSITION 195 side out, and the exterior surface will then digest and the stomach respire. In such cases natural selection might spe- cialise, if any advantage were thus gained, the whole or part of an organ, which had previously performed two functions, for one function alone, and thus by insensible steps greatly change its nature. Many plants are known which regularly produce at the same time differently constructed flowers ; and if such plants were to produce one kind alone, a great change would be effected with comparative suddenness in the char- acter of the species. It is, however, probable that the two sorts of flowers borne by the same plant were originally dif- ferentiated by finely graduated steps, which may still be followed in some few cases. Again, two distinct organs, or the same organ under two very different forms, may simultaneously perform in the same individual the same function, and this is an extremely im- portant means of transition: to give one instance,—there are fish with gills or branchiae 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 vas- cular 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 aerial rootlets ; these three means are usually found in distinct groups, but some few species exhibit two of the means, or even all three, combined in the same in- dividual. In all such cases one of the two organs might readily 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 swimbladdcr in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely, flota- tion, may be converted into one for a widely different pur- pose, namely, respiration. The swimbladdcr has, also, been worked in as an accessory to the auditory organs of certain fishes. All physiologists admit that the swimbladdcr is homol-