In man the greater part of the alimentary canal is composed of a tube of small diameter—the small intestine—which is continued onwards as a more capacious one, the large intestine. These two are not simple continuations one of the other, but the former enters the latter obliquely, the nearer end of the large intestine remaining free as the "blind gut" or cœcum. In the Swallow and Sparrow, as in all the Passeres, instead of there being a single cœcum at the place of junction of the two intestines, there are two. These are not found in the Swifts, nor in the Humming Birds.
In the Swallow, the Sparrow, and all their true allies, it is always the case that the tendons which contract up the last joints of the toes are so arranged that the birds have the power of folding the toe which corresponds to our great toe (the one directed backwards) without moving any of the others. In the Swift, however, whenever the great toe (the hallux) is fully flexed, it is impossible that the other toes should remain opened out, because the two muscles which act on one and the other are bound together by a tendonous band.
In the Swallow, the Sparrow, and most singing birds the number of feathers in the tail is twelve. In the Swifts and Humming Birds the number is always ten—another important difference.
In the Swallow also, as in all the passerine birds, there is a slender muscle running through the thin triangular membrane of the wing between the arm and the fore arm, which is quite peculiar in the manner of insertion or attachment, no other birds possessing the same arrangement. In the Swift this muscle terminates in quite a different manner, here again resembling the Humming Birds exactly.
Taking these several characters into consideration, and realizing how little they are susceptible, on account of their deep-seatedness, to the influence of slight external changes in the mode of life of the species, we are inevitably driven to the conclusion that their weight is overwhelmingly greater than that of the superficial similarity which is so readily brought about by the similarity of the circumstances under which the two species are accustomed to live, and that the resemblances between them are, so far as their constitutions are concerned, dependent only on the fact that they both have—with different pedigrees—arrived at a superficial similarity in contour because they subsist exclusively on the same food.
