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treous substance。 With these facts； here far too briefly and imperfectly given； which show that there is much graduated diversity in the eyes of living crustaceans； and bearing in mind how small the number of living animals is in proportion to those which have become extinct； I can see no very great difficulty （not more than in the case of many other structures） in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane； into an optical instrument as perfect as is possessed by any member of the great Articulate class。
He who will go thus far； if he find on finishing this treatise that large bodies of facts； otherwise inexplicable； can be explained by the theory of descent； ought not to hesitate to go further； and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection； although in this case he does not know any of the transitional grades。 His reason ought to conquer his imagination； though I have felt the difficulty far too keenly to be surprised at any degree of hesitation in extending the principle of natural selection to such startling lengths。
It is scarcely possible to avoid comparing the eye to a telescope。 We know that this instrument has been perfected by the long…continued efforts of the highest human intellects； and we naturally infer that the eye has been formed by a somewhat analogous process。 But may not this inference be presumptuous？ Have we any right to assume that the Creator works by intellectual powers like those of man？ If we must compare the eye to an optical instrument； we ought in imagination to take a thick layer of transparent tissue； with a nerve sensitive to light beneath； and then suppose every part of this layer to be continually changing slowly in density； so as to separate into layers of different densities and thicknesses； placed at different distances from each other； and with the surfaces of each layer slowly changing in form。 Further we must suppose that there is a power always intently watching each slight accidental alteration in the transparent layers； and carefully selecting each alteration which； under varied circumstances； may in any way； or in any degree； tend to produce a distincter image。 We must suppose each new state of the instrument to be multiplied by the million； and each to be preserved till a better be produced； and then the old ones to be destroyed。 In living bodies； variation will cause the slight alterations； generation will multiply them almost infinitely； and natural selection will pick out with unerring skill each improvement。 Let this process go on for millions on millions of years； and during each year on millions of individuals of many kinds； and may we not believe that a living optical instrument might thus be formed as superior to one of glass； as the works of the Creator are to those of man？
If it could be demonstrated that any complex organ existed； which could not possibly have been formed by numerous； successive； slight modifications； my theory would absolutely break down。 But I can find out no such case。 No doubt many organs exist of which we do not know the transitional grades； more especially if we look to much…isolated species； round which； according to my theory； there has been much extinction。 Or again； if 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 extremely 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。 Numerous 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 might easily specialise； if any advantage were thus gained； a part or organ； which had performed two functions； for one function 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 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 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 perform 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 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 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 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 difficulty 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 animals having true lungs have descended by ordinary generation from an ancient 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 of food and drink which we swallow 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 branchiae 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 branchiae might have been gradually worked in by natural selection for some quite distinct purpose： in the same manner as； on the view entertained by some naturalists that the branchiae and dorsal scales of Annelids are homologous with the wings 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 flight。
In considering transitions of organs； it is so import