It has been thought. by Darwin, Wallace, Lub bock, and others that the markings of flowers, such as lines or spots converging to the centre or nectar, serve as guides to insects;. but this is not necessarily the ease, since salvia, wistaria, clover, and other flowers which are each of some one color attract bees equally well. II. Miller has observed that the gentians of the lowlands are visited chiefly by bees, while those of the high Alps are adapted only to butterflies. Most species of Rhinanthus. to which the 'yellow rat tle' of Europe belongs, are what Wallace calls 'bee-flowers.' but one species confined to the high Alps of Switzerland has been adapted to be fertilized by butterflies only. It is now known that flowers did not appear until the Mesozoic age; they are not known to have existed in the Carboniferous. period. Composite flowers, allied to the sunflowers, have been discovered in the Cretaceous clays of New Jersey, and it is prob able that our large and irregular flowers possibly date back to the Cretaceous period. This was the time when insects that visit flowers also originated. No pollen-eating beetles, no moths. butterflies, wasps, or bees are known to have existed in the Carboniferous. nor before the middle or end of the Mesozoic; it seems prob able, therefore. that flowers and the insects which visit them appeared at about the same epoch.
We may correlate with this view the theory of Henslow that the irregular flowers, such as those of the pea. bean. etc., are due to the inter mittent mechanical stimulus resulting from the visits of butterflies. moths. and bees. lie de pends on the Lamarekian factors of use and disuse, and use-inheritance, to account for the beautifully irregular forms of the papilionaceolis and labiate flowers, as well as the singular and gorgeous flowers of the ()raids., dispensing with the theory of natural selection. However this may be, in the beginning flowers were most probably small, regular in shape, inconspicuous, and kelf-fertilized. Henslow, and also Wallace, take the view that many inconspicuous and im perfect flowers. including those that are wind fertilized, such as plantains, nettles. sedges. and grasses. do not represent primitive or undevel oped forms. but have. through the neglect of in sects, become degenerate types, derived from more perfect forms which were originally adapt ed to insect fertilization.
F'crtili ulioe of Flowers by Birds.—While iu the north temperate zone insects appear to be the chief means of cross-fertilization of flowers, where this is not cawed by the wind, in the tropics and Southern Hemisphere, Wallace tells u.s, birds have in many cases led to modifications in the form and colors of flowers. Humming birds are active in performing this office, fertiliz ing many such blossoms as the passion-tlow(Ts, trumpet-flowers. fuchsias. and lobelias. The Salvia splendens of Mexico is especially adapted to their visits: and in the Andes and in Chile, where these birds are extremely abundant. many kinds Of red tubular flowers, often of great size, are apparently adapted to the long, slender bills of these hummers. The most extraordinary
adapt atbm to bird-fertilization are the of Maregravia, in whiell. says Wallace, the pedi eels and bracts of the terminal portion of a pendent hunch of flowers have been modified into pitchers which secrete nectar and attract in sects, while birds feeding on the nectar or in sects have the pollen of the overhanging flowers dusted on their backs, and carrying it to other flowers, thus cross-fertilizing them. in the east ern tropics the sun birds lake the place of the hummers, and they are aided by the flower peckers. (See Dtc,Eum.) In the „1ustralian region there are two flower-feeding groups—the honey-suckers (Meliphagithe) and the brush tongued lo•ies (Trichoglossidi•).
In New Zealand the use of animal life in fer tilizing thINVCIS is seen in the case of a country which is remarkably poor in specie,: of insects. especially bees and butterflies; yet, it has been shown by local botanists that no less than one fourth of all the flowering plants of those islands are incapable of fertilization. and thus wholly dependent on animal life for the continuance of the species.
_tdranluycs of I ntererossing.—This brief ac count of cross-fertilization of flowers shows, as pointed out by Darwin and others, that we probably owe to the visits of insects and birds the peculiar mid varied forms and structures of our most beautiful and attraetive flowers, and that as the result of intererossing the size, height, vigor, and fertility of the race or species is en hanced. Yet with plants, as we shall see is the case with animals, the mere act of inter crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution. owing to their progenitors having been subjected during several generations to slightly different condi tions.
Cross-Fertilivttion in Animals and Man.—ln tererossing in animals, as among plants, may, within due limits, be beneficial. These limits are confined to the same variety, race, or stock. If individuals of widely different races or breeds intermix, the result is degeneracy and steril ity, the outcome of such unions being of the same nature as hybrids between different though allied species. As Darwin states: "After plants have been propagated by self-fe•tilization for -everal generations, a single cross with a fresh stock restores their pristine vigor; and we have a analogous result with our domestic animals." This will apply to man also. When the French aristocracy was. as the result of the Heyolntion, broken up and forced to intermarry with the bourgeoisie, the result was an increase in the population and additional vigor in the race. An old family in its decline may be re juvenated and restored by intermarriage with a more vigorous, even if a coarser, stock or strain. The population of our cities is main tained by the constant influx of fresh blood from the rural districts.