FOOD RELATIONS OF AQUATIC ANIMALS. ARROWS POINT FROM ORGANISMS EATEN TO THOSE DOING THE EATING "Let us assume that because of some unfavorable conditions in a pond during their breeding period the black bass decreased markedly. The pickerel, which devour young bass, must feed more extensively upon insects. The decreased number of black bass would relieve the drain upon the crayfishes, which are eaten by bass; crayfishes would accordingly increase and prey more heavily upon the aquatic insects. This combined attack of pickerel and crayfishes would cause insects to decrease and the number of pickerel would fall away because of the decreased food supply. Meanwhile the bullheads, which are general feeders and which devour aquatic insects, might feed more extensively upon mollusks because of the decrease of the former, but would probably decrease also because of the falling off of their main article of diet. We may thus reasonably assume that the black bass would recover its numbers because of the decrease of pickerel and bullheads, the enemies of its young." 9 5 The great advantage which animals have over plants as regards food grows out of their power of motion. If 5 SHELFORD, V. E. o. c., p. 70. the supply is inadequate in one place they may go in search of another, though it is to be recognized that the search is not always successful. In return for the contributions made by plants to animals, the latter often perform services of highest importance to plants. Chief of these is perhaps the fertilizing of the flowers. Many varieties produce on one plant flowers having only pistils or stamens, and the pollen must be carried either by the wind or by insects. Other flowers are so shaped that only by some outside agency can the pollen reach the pistil even though both are present. The fig has its flowers inside a tube into which certain insects must crawl. Every one has watched the bees gathering honey in the flowers and incidentally and unwittingly getting on their bodies pollen which they carry to other plants and thus cross-fertilize them. Without such cooperation many species would perish. Thomson in happy fashion enlarges one of Darwin's illustrations: "Plants and animals remote in the scale of nature are bound together by a web of complex relations. . . . I have also found that the visits of bees are necessary for the fertilization of some kinds of clover,- thus 100 heads of red clover (Trifolium pratense) produced 27,000 seeds, but the same number of protected heads produced not a single seed. Humble bees alone visit red clover, as other bees cannot reach the nectar. We know that the red clover imported to New Zealand did not bear fertile seeds till humble bees were also imported. The number of humble bees in any district depends in great measure on the number of field mice which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble bees, believes that more than two-thirds of them are thus destroyed all over England.' Now the number of mice is largely dependent, as every one knows, on the number of cats; and Colonel Newman says: 'Near villages and small towns I have found the nests of humble bees more numerous than elsewhere and this I attribute to the number of cats that destroy the mice.' Thus we may say with Darwin that next year's crop of purple clover is influenced by the number of humble bees in the district, which varies with the number of field mice, that is to say, with the number of cats." This involves a tremendous amount of labor. A red clover blossom contains less than one-eighth grain of sugar. There are 7,000 grains in a pound. The bee must visit some 56,000 clover heads inserting its proboscis into some 60 florets on each head thus repeating the operation 3,360,000 times to get a pound of honey. "More than two thousand years ago Herodotus observed a curious custom in Egypt. At a certain season of the year, the Egyptians went into the desert, cut off branches from the wild palms, and bringing them back to their gardens, waved them over the flowers of the date-palm. Why they performed this ceremony they did not know; but they knew that if they neglected it, the date crop would be poor or wholly lost. Herodotus offers the quaint explanation that along with these branches there came from the desert certain flies possessed of a 'vivific virtue' which somehow lent an exuberant fertility to the dates. But the true rationale of the incantation is now explained. Palm trees, like human beings, are male and female. The garden plants, the date bearers, were females; the desert plants were males; and the waving of the branches over the females meant the transference of the fertilizing pollen dust from the one to the other." 997 6 THOMSON, J. A. o. c., p. 53. 7 DRUMMOND, HENRY. Ascent of Man, p. 242. the supply is inadequate in one place they may go in search of another, though it is to be recognized that the search is not always successful. In return for the contributions made by plants to animals, the latter often perform services of highest importance to plants. Chief of these is perhaps the fertilizing of the flowers. Many varieties produce on one plant flowers having only pistils or stamens, and the pollen must be carried either by the wind or by insects. Other flowers are so shaped that only by some outside agency can the pollen reach the pistil even though both are present. The fig has its flowers inside a tube into which certain insects must crawl. Every one has watched the bees gathering honey in the flowers and incidentally and unwittingly getting on their bodies pollen which they carry to other plants and thus cross-fertilize them. Without such cooperation many species would perish. Thomson in happy fashion enlarges one of Darwin's illustrations: "Plants and animals remote in the scale of nature are bound together by a web of complex relations. . . . I have also found that the visits of bees are necessary for the fertilization of some kinds of clover,- thus 100 heads of red clover (Trifolium pratense) produced 27,000 seeds, but the same number of protected heads produced not a single seed. Humble bees alone visit red clover, as other bees cannot reach the nectar. ... We know that the red clover imported to New Zealand did not bear fertile seeds till humble bees were also imported. The number of humble bees in any district depends in great measure on the number of field mice which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble bees, believes that more than two-thirds of them are thus destroyed all over England.' Now the number of mice is largely dependent, as every one knows, on the number of cats; and Colonel Newman says: 'Near villages and small towns I have found the nests of humble bees more numerous than elsewhere and this I attribute to the number of cats that destroy the mice.' Thus we may say with Darwin that next year's crop of purple clover is influenced by the number of humble bees in the district, which varies with the number of field mice, that is to say, with the number of cats." 6 This involves a tremendous amount of labor. A red clover blossom contains less than one-eighth grain of sugar. There are 7,000 grains in a pound. The bee must visit some 56,000 clover heads inserting its proboscis into some 60 florets on each head thus repeating the operation 3,360,000 times to get a pound of honey. "More than two thousand years ago Herodotus observed a curious custom in Egypt. At a certain season of the year, the Egyptians went into the desert, cut off branches from the wild palms, and bringing them back to their gardens, waved them over the flowers of the date-palm. Why they performed this ceremony they did not know; but they knew that if they neglected it, the date crop would be poor or wholly lost. Herodotus offers the quaint explanation that along with these branches there came from the desert certain flies possessed of a 'vivific virtue' which somehow lent an exuberant fertility to the dates. But the true rationale of the incantation is now explained. Palm trees, like human beings, are male and female. The garden plants, the date bearers, were females; the desert plants were males; and the waving of the branches over the females meant the transference of the fertilizing pollen dust from the one to the other."7 6 THOMSON, J. A. o. c., p. 53. 7 DRUMMOND, HENRY. Ascent of Man, p. 242. |