The problems of analogous variation or parallelism without homology are very often met with among fishes. In shallow, swift brooks in all lands there are found small fishes which hug the bottom--large-finned, swift of movement, with speckled coloration, and with the air-bladder reduced in size. In the eastern United States these fishes are darters, dwarf perches; in northern India they are catfishes; in Japan, gobies or loaches; in Canada, sculpins; in South America, characins. Members of various groups may be modified to meet the same conditions of life. Being modified to look alike, the thought of mutual affinity is naturally suggested, but in such cases the likeness is chiefly external. The internal organs show little trace of such modifications. The inside of an animal tells what it really is, the outside where it has been. In other words, it is the external characters which are most readily affected by the environment. Throughout all groups of animals and plants, there are large branches similarly affected by peculiarities of conditions. This is the basis of the law of "Adaptive Radiation." Prof. H. F. Osborn thus states this law: "It is a well-known principle of zoological evolution that an isolated region, if large and sufficiently varied in its topography, soil, climate, and vegetation, will give rise to a diversified fauna according to the law of adaptive radiation from primitive and central types. Branches will spring off in all directions to take advantage of every possible opportunity of securing food. The modifications which animals undergo in this adaptive radiation are largely of mechanical nature; they are limited in number and kind by hereditary stirp or germinal influences, and thus result in the independent evolution of similar types in widely separated regions under the law of parallelism or homoplasy." FOOTNOTES: [63] This chapter and the next are in substance reprinted from an essay published by the present writer in a volume called Science Sketches. A. C. McClurg & Co., Chicago. [64] _Salvelinus fontinalis_ Mitchill. [65] _Semotilus atromaculatus_ Mitchill. [66] _Notropis cornutus_ Rafinesque. [67] _Catostomus commersoni_ (Lacépède). [68] _Ameiurus melas_ Rafinesque. [69] _Eupomotis gibbosus_ Linnæus. [70] _Etheostoma flabellare_ Rafinesque. [71] _Rhinichthys atronasus_ Mitchill. [72] On Fishes from Tennessee River, Alabama. American Journal of Science and Arts, xvii., 2d series, 1854, p. 26. [73] On the Distribution of Fresh-water Fishes in the Alleghany Region of Southwestern Virginia. Journ. Acad. Nat. Sci., Phila., 1868, pp. 207-247. [74] Introduction to the Study of Fishes, 1880, p. 211. [75] The table below shows approximately the composition of the fresh-water fish fauna of Europe, as compared with that of North America north of the Tropic of Cancer. Families. Europe. N. America.