CONVERGENCE. Cases often occur where two animals of different groups, with a different ancestry and affinities, but with similar habits, so closely resemble each other that not only the ordinary observer, but the experienced naturalist, is deceived by their close resem blance. A familiar example is the whale, which so resembles a fish that many it is even supposed to be one. Now, the whale is a mal, bringing forth its young alive, and suck ling it. The cetaceans form an order by them selves. There are strong reasons for believing that they are the descendants of some group of land vertebrates which walked on all fours, but which, perhaps driven by competition, were forced to adopt marine life, and became won derfully adapted to an aquatic life, during this process losing by disuse their hind limbs, while the fore legs became converted into fins. By adaptation to the same medium, a fish and a whale have a similar shape and a strong super ficial resemblance. The same is the case with certain extinct whale-like lizards, such as the ichthyosaurs. These, with the plesiosaurs, are now supposed to have descended from some earlier four-footed terrestrial reptiles, which, becoming adapted to oceanic life, assumed a fish-like fomi.
Cases of convergence resulting from similar burrowing habits are seen in the Amphibia and reptiles. Certain amphibians (Cavilia) and several extinct Carboniferous forms, have lost their limbs by disuse; they are worm-like, from adopting the habits of earthworms. Among the lizards the glass-snake (Ophiosaurus) and a few other forms have lost their legs in conse quence of burrowing in the sand. There is a species (Bipes) in which a pair of legs are re tained. Snakes have evidently descended from four-legged forms, the boas still retaining ves tiges of the hind legs. It is not an easy matter to separate some of the legless litards from small boas, owing to the convergence in their mode of life.
The thousands and tens of thousands of the boring larvm of insects, belonging to quite different groups, have strikingly similar forms owing to their similar habits; thus the headless and apodous maggots of flies resemble those of ants, wasps and bees. Among jumping mam mals, the kangaroo, the jerboa and jumping mice have similar large muscular hind legs, with a reduction in the number of toes, although they belong to different sub-classes or orders. The kangaroo is a marsupial and we have marsupial or kangaroo rats and mice which can be sepa rated only by an expert from ordinary rodents. The koala mimics the bear, the pouched wea sels look like genuine weasels, and so on.
A multitude of other examples can be cited to illustrate the effects of convergent habits, or the influence of similar conditions of life, or adaptation to such and such surroundings. It is most probable that many, if not the large majority, of the cases of mimicry among butter flies and other insects generally attributed to the action of natural selection, are examples of convergence, resulting from exposure to simi lar physical conditions of light, temperature, etc., which have produced similar styles of coloration, outlines in their wings, etc.
While convergence is not in itself a primary factor of organic evolution, use and disuse are such factors, and convergence in habits or triodes of life, resulting in use or disuse_ of parts, have had much to do with the evolutron of such extremely specialized groups as the whales, the snakes, the plesiosaurs and ichthy osaurs, as well as other minor groups of animals.
Convergence is of relatively tare occurrence in plants. However, the phenomenon of hetet-6 spory in Pteridophytes is usually supposed to have originated separately in the several groups.