In man and the highest apes the caecum has a vermiform appendix, as it has also in the wombat! Also the similar forms presented by the crowns of the teeth in some seals, in certain sharks, and in some extinct Cetacea may be referred to; as also the similarity of the beak in birds, some reptiles, in the tadpole, and cuttle-fishes.

As to entire external form, may be adduced the wonderful similarity between a true mouse (_Mus delicatulus_) and a small marsupial, pointed out by Mr.Andrew Murray in his work on the "Geographical Distribution of Mammals," p.

53, and represented in the frontispiece by figures copied from Gould's "Mammals of Australia;" but instances enough for the present purpose have been already quoted.
Additional reasons for believing that similarity of structure is produced by other causes than merely by "Natural Selection" are furnished by certain facts of zoological geography, and by a similarity in the mode of variation being sometimes extended to several species of a genus, or even to widely different groups; while the restriction and the limitation of such similarity are often not less remarkable.

Thus Mr.Wallace says,[62] as to local influence: "Larger or smaller districts, or even single islands, give a special character to the majority of their Papilionidae.

For instance:--1.
The species of the Indian region (Sumatra, Java, and Borneo) are almost invariably smaller than the allied species inhabiting Celebes and the Moluccas.2.The species of New Guinea and Australia are also, though in a less degree, smaller than the nearest species or varieties of the Moluccas.
3.