tain spotted fever in man. The numerous mites which infect plants and animals are mostly diminutive, and some of them, such as the species which

causes the itch in human beings, are almost invisible to the naked eye.

Among the insects, parasitism assumes a number of very diverse forms. Perhaps the extreme of degeneracy is found in the wingless and almost immobile scale bugs. Parasitism has

not infrequently led to degeneracy among insects, but there are numerous species in which it does not have this effect. In many cases

a

FIG. 151-The scurfy bark-louse: a, female

scale bugs; b, male scales; c, female scale enlarged; d, male scale enlarged. (After Howard.)

this is probably due to the fact that parasitism is confined to the larval stage of development. The flies of the family Tachinidæ lay their eggs upon the bodies of other insects, and especially upon larvæ. The young larval parasites that hatch

from the eggs bore into the body of their host and live upon its tissues. Somewhat similar habits are found in the ichneumon flies and their relatives among the Hymenoptera. The larvæ of many of the smaller species live within the eggs of other insects. The ichneumon Pimpla conquisitor pierces the cocoon of the tent caterpillar and deposits her eggs upon the pupa, and the larvæ, when they hatch out, proceed to devour the helpless pupa at their leisure. Another ichneumon, Thalessa lunator, has a remarkably long ovipositor with which she bores into wood until she reaches the burrow of the

larva of a wood-boring sawfly, Tremex. When the egg hatches the young larva crawls along the burrow until it reaches its vic

FIG. 153-The ichneumon fly, Thalessa lunator, in the act of boring into wood with her ovipositor. (After Riley.)

tim, which it proceeds to devour. The solitary wasps

may be regarded as parasitic in their larval state, since the mother wasp usually lays her egg upon her prey without killing it, thus allowing her larva to feed upon the tissues of a living organism.

The insects whose larvæ lead a parasitic life are mostly very active, alert, and discriminating creatures which exhibit complex and highly specialized behavior in providing for

the welfare of their progeny. Parasitism of this particular kind would seem, therefore, to favor the development of more highly organized forms. Degeneracy

would be quite inconsistent with the exacting requirements which it imposes.

We have described only a few of the more common kinds of associations between organisms, but there are many other kinds of which we must omit any mention. The plants and animals of any limited area have numerous interrelations and may be said to constitute a sort of society. Insects and flowers are united in a relation of mutual service. Herbivores devour plants, and carnivores prey upon herbivores. Birds eat the seeds of plants and destroy insects that are injurious to plant life. Parasites

FIG. 154 Lysiphlebus, in the act of depositing an egg within the body of an aphis. (After Webster.)

attack all kinds of organisms, and bacteria and fungi resolve the dead bodies of plants and animals into simpler constituents that constitute the food of plants. A great increase of insect life favors the multitudes of spiders, and insectivorous birds and mammals, and thus the undue preponderance of insects is checked. The organic life of a given locality subsists in a sort of balance which tends to be maintained because disturbances of the balance tend sooner or later to bring about their own remedy. This may be well illustrated by a passage from Darwin. After speaking of the influence of cattle in checking the growth of Scotch fir, he remarks:

In several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies-then cattle and horses would become feral, and this would certainly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onwards in everincreasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another.

REFERENCES

ADAMS, C. C., Guide to the Study of Animal Ecology. N. Y., Macmillan,

1913.

BORRADAILE, L. A., The Animal and Its Environment. London, Frowde, Hodder, and Stoughton, 1923.

CHANDLER, A., Animal Parasites and Human Disease (2nd ed.). N. Y., Wiley, 1922.

FANTHAM, H. B., STEPHENS, J. W. W., AND THEOBALD, F. V., The

Animal Parasites of Man. N. Y., Wood, 1920.

HEGNER, R. W., Cort, W. W., AND ROOT, F. M., Outlines of Medical Zoology. N. Y., Macmillan, 1923.

AND TALIAFERRO, W. H., Human Protozoology. N. Y., Macmillan, 1924.

HERMS, W. B., Medical and Veterinary Entomology. N. Y., Macmillan, 1923.

KEEBLE, F., Plant-Animals: A Study in Symbiosis. Cambridge University Press, 1912.

REINHEIMER, H., Symbiogenesis. Philadelphia, Knapp, Drewitt, 1915. RIVAS, D., Human Parasitology. Philadelphia, Saunders, 1920. SHELFORD, V. E., Animal Communities in Temperate America. University of Chicago Press, 1913.

VAN BENEDEN, E., Animal Parasites and Messmates. N. Y., Appleton, 1876.

CHAPTER XIII

REGENERATION, EXPERIMENTAL EMBRYOLOGY, AND THE REGULATION OF ORGANIC FORM

The activities described in previous chapters are mainly those which form a part of the usual routine of living. But the course of life is not always suffered to run on smoothly. Living beings are subjected to mutilations, distortions, and many other abnormal modifications, and the ways in which they meet these various mishaps afford some of the most striking manifestations of the phenomena of life.

Everyone is familiar with the fact that many organisms are able to regenerate lost parts. A lizard may form a new tail, and a newt may regenerate a

lost leg or eye. The power of regeneration is generally greater in the lower and simpler forms of life. The trumpet-shaped infusorian Stentor may be cut into several pieces, and each piece, provided it contains a part of the long, moniliform nucleus, will transform itself into a new Stentor similar to the original one except in size.

Lillie has shown that a

fragment as small as one

sixty-fourth of the original Stentor may form a new individual.

The fresh-water Hydra has long been a favorite subject for the study of regeneration ever since the Abbé Trembley per