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Show 1'70 DIFFICULTIES ON THEORY. tCnAP. VI. we look to an organ common to all the members of a , large class, for in this latter case the organ must have been :first formed at an extrernely remote period, since which all the many members of the class have been developed ; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ancestral forms, long since become extinct. We should be extremely cautious in .concluding that an organ could not have been formed by transitional gradations of some kind. Nun1erous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus the alimentary canal respires, digests, and excretes in the larva of the dragon-fly and in the fish Cobites. In the Hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire. In such cases natural selection ·rnight easily specialise, if any advantage were thus gained, a part or organ, which had performed two functions, for one fimction alone, and thus wholly change its nature by insensible steps. Two distinct organs sometimes perform simultaneously the same function in the same individual; to give one in~ stance, there are fish with gills or branchire that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladd.ers, this latter organ having a ductus pneumaticus for its supply, and being divided by highly vascular partitions. In these cases, one of the two organs might with ease be modified and perfected so as to perfor1n all the work by itself, being aided during the process of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be quite obliterated. The illustration of the swim bladder 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 wholly different purpose, namely, respiration. The swimbladder has, also, been worked in as an accessory to the auditory organs of certain fish, or, for I do not know which view CRAP. VI.] TRANSITIONS OF ORGANS. 171 is now generally held, a part of the auditory apparatus has been .worked in as a complement to the swimbladder. 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 seems to me to be no great diffiCltlty in believing that natural selection has actually converted a swimbladder into a lung., or organ used exclusively for respiration. I can, Indeed, hardly doubt that all vertebrate animal~ having true lungs have descended by ordinary genel'ation from an anment prototype, of which we know nothing, furnished with a floating apparatus or swimbladder. We can thus, as I infer from Professor Owen's interesting description of these parts, understand the strange fact that every particle Qf food and drink which we swallo·w has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher Vertebrata the branchire have wholly disappeared- the slits on the sides of the neck and the loop-like course of the arteries still marking in the embryo their former position. But it is conceivable that the now utterly lost branchire might have been gradually worked in by natural selection for some quite distinct purpose : in the sa~e manner as, on ~he view entertained by some naturalists that the branchire and dorsal sc.ales of Annelids ~r~ homologous with the win~s and wing-covers of insects, It IS probable that organs whiCh at a very ancient period served for respiration have been actually converted into organs of fiigh t. In considering transitions of organs, it is so important to be~r in mind the probability of conversion from one fun0tion to another, that I will give one more instance. Pedunculated cirripedes have two 1ninute folds of skin called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are h.atched within the sack. These cirripedes have no b;anch1re, the whole surface of the body and sack, including the small frena, serving for respiration. The Balanidre or sessile cirripedes, on the other hand, have no |