carry leaves into the soil and thus aid in producing the humus. Thomson states that in Yorubaland on the west coast of Africa it is estimated that every particle of soil is brought to the surface once in twenty-seven years. Said Darwin: "It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures.” 2 Ants and other insects perform similar

services.

Another type of dependence must be noted. The digestive tract of all the higher animals is densely populated, from a few hours after birth till death by great numbers of micro-organisms. Says Herter: "There is not the least doubt that in some way a rather intricate way. these normal and dominant flora of the intestine exert a protective action on the whole body. This protective action is complex in operation, but consists at least in part of a preemptive effect on the intestinal domain, by which other types of bacteria are in large degree excluded. It is impossible to avoid introducing into the digestive tract many bacteria which would prove undesirable permanent tenants, and these are successfully discouraged from gaining a foothold by the motile biological activities of the more permanent and better adapted bacterial guests. If the native flora are destroyed or injured "wild" types may be introduced and this Metchinikoff, one of the most eminent biologists, believed to be a common cause of suffering and premature death. idea is not universally accepted, and many still believe this natural flora to be neutral in influence rather than helpful. With reference to other types we must recognize

" 3

This

2 THOMSON, J. A. Darwinism and Human Life, pp. 56-59. 3 HERTER, C. A. Biological Aspects of Human Problems, p. 130.

probably that living within our bodies, by their life and death many poisons are produced which are directly injurious to the body. From this standpoint health depends upon a nice adjustment between the natural defenses of the body and the attacks of the invaders. Some of the evil effects will be considered later.

In the preceding chapter we saw that the presence of life in any area depended upon certain physical conditions. It must now be noted that the presence of any of the higher types of life depends also upon the presence of certain other types. In other words, there is a series of interrelations reaching from the lowest to the highest forms. Wherever conditions are favorable the mass of the lowest forms is almost beyond comprehension. I have seen a small pool in Mississippi containing much decaying matter so densely inhabited that the surface was kept in constant motion with a bubbling as of a small brook. To this aggregation of small plants and animals in water we give the collective name plankton. In Lake Michigan Shelford found 11.5 c.c. of this plankton in each cubic meter of water. He states that if the inhabitants of Chicago drink daily 2,000,000 quarts of unfiltered water they imbibe therewith ten solid quarts of plants and animals. He reports that in some of the European lakes ten times this amount is found. In the Illinois River 71.36 c.c. were found, and in an Indiana lake 684 c.c. The largest amount was present from April to June, the least in December and January. The abundance of plant growth depends upon the amount of all the necessary foodstuffs. If any one is absent growth is stopped. As a rule the older and quieter the body of water the greater the plant life. Within certain limits this is also true of animal life. The 4 SHELFORD, V. E. o. c., p. 67.

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 zoology 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."

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.

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.