Page:The New International Encyclopædia 1st ed. v. 07.djvu/375

* EVOLUTION. 3-.'; EVOLUTION. Lexington, Va. In Europe this is an exceedingly variable species, but already of the 125 Virginian varieties found by Mr. Cockerell, 67 are new and unknown in Europe. But by far the strongest and clearest evidence of lln' means by which species are originated are afforded by Dr. J. A. Allen in the case of our American birds and mammals, his resull having been based on prolonged studies made upon a ;i - 1 number of specimens from different localities. Our birds are found in passing from the Atlantic to the Pacific coas! to varj in gen eral size, in the size of the peripheral parts (wings, etc.), and in color, thus varying with latitude or longitude. There is an increase in size from the south northward, not only in in- dividuals, but generally, though there are some exceptions. The largest species of eaeli genus and family are northern, as in the cases of the fox and wolf, the latter being one-fifth larger in Alaska than are southern species of their kind. In the ease of those birds which breed from New England to Florida, the southern ones are small- er and differ in color. Mammals and birds, in their southerly examples, have larger ears and feel, and the cattle have larger horns. The hares have less furry ears and naked soles; the sage-brush hare has longer ears southward; so with the large long-eared 'jack-rabbit.' In birds hill, claws, and tail are larger in southern species and all the largest-billed birds live in the tropics. This, however, is to be observed in sparrows, blackbirds, crows, thrushes, wrens, and war- blers, in the quail, meadow-lark, and flicker. In Florida, forms with slender bills common to thai State and to the North have beaks si ill more slender, longer, and decurved. Those with a short conical bill have thicker and longer bills than their northern relatives, though the birds themselves are smaller. It is so with the tail — the size of the body is the same as in the North, while the tail is proportionately larger and longer. The color in mammals, as the red squirrel, changes in going southward from pale yellow or fulvous to rufous. Except three species, all squirrels living north of Mexico have the lower parts of the body white, while those inhabiting tropical Mexico have the lower parts fulvous, deep golden, orange, or even dark brownish-red. In lords the colors are so much stronger and darker in southern forms that they might with their smaller size and larger bills be regarded as distinct species. The blue-jay. cardinal, and other birds have, in the South, a more brilliant and intense hue: some species are mere black and red. In crossing from the Atlantic to the Pacific coast, Allen observes that there are three phases of color. On the Atlantic coast the birds are bright and strongly colored: on the great plains they are pallid, owing to the dryness; and on the humid, heavily wooded Pacific coast the hues are deep-colored or piceous, both in birds and mammals. The same obtains in the Old World. The marsh-tit of Europe in warm, rainy regions has its browns intensified: in dry. sandy districts the plumage is paler; in the Arctic regions it varies in paleness, and in Kamchatka it is al- most white (Dixon). The birds of the Galapagos Islands differ from their nearest allies of the South American mainland in their larger bills, shorter wings, longer tails, and darker colors. ide il'i-. each of these islands has it- local species or ariet ies, « hii island in t be othi Man} nih'i examples could be given, but 1 1 "ii ha beei tated to provi that in the pa as wi i climate have had an all-powerful influence in il igination of species. To this factor, together with miei lion and geographical i olation, we may attribute a very la i ge pro] ' a n specie ol plant and animal life, and also <>i man I. iih] Seasonal Di mohphism. < irdinarj ua I di morphism is where there are two forms of one sex, the cause oi such i modification being un- known, in seasonal dimorphism, however, the cause is due to changes of temperature. Thus by subjecting tl tlid of lowland butter- flies to prolonged cold in ice-che I Wei mann proved that northern or alpine species are climatic or seasonal varieties. W. II. Edwards ha-, shown that two of the four polymorphic forms of Papilio Ajar i i.e. Walshii and Tela nnmiilfs) emerge- from winter chrysalids, and Pfijiilio Ma reel I us emerges from a second brood of summer chrysalids. Cases of seasonal dimorphism frequently oc- cur among tropical butterflies. While in the north or temperate zone we have winter and sum mer forms, in the tropics of India and of Africa there are wet-season and dry-season forms. Tt is curious that the difference between the two forms principally consists in the fact that one looks on the under side like a dry leaf, while the other is marked with eye-like spots or ocelli. Their identity has been proved by raising both forms from the same batch of eggs. This case appears to be due to the direct action of the season, in the dry form to dryness and heat, in the wet form to the moisture and coolness of the wet season. Weismann, however, maintains that these changes of climate or season are 'only the stimulus, not the actual causes,' the latter being the processes of selection, a quite hypothetical cause, although twenty years ago he attributed the change to the effect of change of temperature. Limitations of space forbid one enlarging on this fascinating theme, but we may briefly refer to the remarkable experiments of Standfuss, who, by subjecting pupa? of Swiss butterflies to hi or cold, has produced artificially true temperature varieties, as follows: (1) Seasonal forms, similar to those known in nature ( Vanessa cardui nlbus, and Papilio Machaon to some extent). (2) Lo- cal forms and races similar to those which occur constantly in certain localities [Vanessa urticos, cardui, and to some extent Papilio Machaon and Vanessa Antiopa). (3) Entirely exceptional forms or aberration-., also occurring from time to time in nature I 1 anessa l<>. cardui, Argynnis, Aglaia). (4) Phylogenetic fon I novi ex- isting, ''but which may either have existed in pas! epochs or may perhaps be destined to arise in the future" (Vanessa to, Antiopa, Atdkmta). The conclusion from this and experiments by others shows, as Standfuss claims, thai such forms are the result of the direct action of a change in the temperatun Effei is of Ch inge of Food. While changes of light, heat or cold, moisture and dryness, are fundamental factors in causing variation, the abundance or scarcity and the r.atnre of the food are an equally potent agency, not only affecting