oldest pond may show an excess of carbon dioxid or too little oxygen, or offer no good breeding place for many species of fish. In such ponds the relations of animals and plants are most interesting. Floating plants gather on the stems of the larger rooted varieties. Hither come snails and other animals seeking food. They too attach themselves to the stems or leaves finding there protection against currents, shelter from excessive light and opportunities for laying their eggs. Some indeed depend more directly upon plants. Certain leaf-eating beetles are aquatic in their early stages. They have no gills, but attach themselves to plants from which they get a direct supply of oxygen even under the water.

So many and so varied are the interrelations between the highest and the lowest forms of life that it is impossible to exaggerate them and almost impossible for one not a close student of zoölogy to realize their extent and significance. Volumes of illustrations could be given. Here space permits reference only to a few of the different types of relationship. Under the title "ecology "great attention is now being given to this study of community life, particularly of animals. It is believed that, under similar conditions the world over, there will be found animals not necessarily the same, but filling the same niches in the scheme.

In the world of water crustaceans are the most important invertebrates, "the entomostraca being from the standpoint of food supply to the water what rooted plants are on the land, one of the things to which food interaction can be traced." This food dependence is well shown by Shelford in the following diagram.

FOOD RELATIONS OF AQUATIC ANIMALS. ARROWS POINT FROM OBGANISMS 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.'

If

The great advantage which animals have over plants as regards food grows out of their power of motion. 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."7

6 THOMSON, J. A. o. c., p. 53.

7 DRUMMOND, HENRY. Ascent of Man, p. 242.

Animals assist greatly in the spread of plants and trees. Along the fence rows appear cherries, sumac, dogwood and junipers whose seeds have been deposited by birds. Squirrels often carry walnuts, chestnuts and acorns considerable distances. Many seeds like burdock or cockleburs catch in the hair of passing animals and are thus transported to new locations.

Every observer has in spring time watched the ants crawling over the buds of flowers and at first doubtless feared lest they do them damage. Closer study shows that the ants are seeking the aphids or plant lice from whose bodies they get a drop of liquid of which they appear very fond, sometimes stroking and caressing the aphid to stimulate the production of the fluid. Hence aphids have been called the slaves or the " cows," as Linnæus names them, of the ants. In any case they directly furnish food or some acceptable stimulant.

The complex social life of bees and ants is so often described that any extended account is unnecessary. It is worth while to indicate some of their methods, however, which vary greatly according to the species. The Amazon ant "exists only by the capture of slaves and in that connection develops the most brilliant warrior talent that we know in the entire animal kingdom. Its mandibles are modified to be solely weapons for killing and are unsuited for domestic occupations; furthermore it has even lost the instinct of feeding by itself and must be fed out of the mouths of its slaves." Wasmann estimates that some 2,000 species of animals are found in association with ants, some as guests, some as slaves, others as parasites. Some beetles "are not only licked by their hosts (first step), but are also fed regularly from their mouths (second step), and finally also the larvæ of these