I

What is human welfare? Deep thinkers have answered this question in a great variety of ways, each trying to catch the elusive essence of the Good, and few succeeding in such wise as others can understand and agree with. In recent years, however, a tendency is observable among philosophers to accept what Bertrand Russell calls the Good Life as the practical expression of human welfare and as the essential element in any valid definition of the phrase. From concern with abstractions, such as other-worldly ideals, the summum bonum, and the greatest good of the greatest number, we are thus brought to a primary regard for individual happiness, for the successful life of the individual. This point of view, once agreed upon, helps us to define human welfare in simple, honest, and scientific fashion and, in consequence, to detect and modify the factors which affect it. Let us agree that welfare requires conditions of life that are reasonably favorable with regard to health, wealth, social relations, and intellectual outlook. Exceptional individuals, to be sure, may fare best under certain handicaps or may require some peculiar favor from fate; but certainly the conditions we have mentioned are, in general, the requisites for living the good life. What is zoology?

The human race is a zoological species; it is one among the almost one million kinds of animals that have been scientifically studied; it is bound up with zoology as with no other science. The objective study of animals and the body of knowledge and doctrine derived from such study must therefore have peculiar importance for mankind, and we are here interested to see some of the ways in which zoology bears upon the phases of existence which we have distinguished as the necessary elements in human welfare. Zoology contributes to human health because the principles of hygiene and normal physiology are zoological and because many diseases are caused or carried by animals; to wealth because many animals are economically useful or injurious; to social relations because human reproduction and heredity are fundamentally animal in nature; to intellectual outlook because zoological theory forms an important part in our modern views of psychology, morals, and progress.

II

The aim of hygiene is to maintain conditions favorable to the normal activity of the human machine. Apart from the fragmentary and often dubious lessons of ordinary experience, this aim is accomplished by relying upon the discoveries of physiology, a branch of zoology which has made most of its important gains through studying the functions of organs in animals. By using other mammals, such as the dog, cat, and guinea pig, medical men can extend very materially the field of experimentation, which is strictly limited (though not entirely closed) when they are dealing with human cases. And the zoological fact that these animals are in many respects similar to the human species makes the results of such experimentation immediately applicable, in many instances, to the study of human physiology, normal or pathological.

In such work it is often necessary to inoculate animals with the germs of human disease, or to practice vivisection upon them, as the only method of understanding what goes on in the human body; and it seems incredible that any one should wish to obstruct this very necessary and remarkably successful form of investigation. Yet it is true that the anti-vivisectionists, obsessed by a sentimentally inverted humanitarianism, have sometimes lost sight of human welfare in their concern for what they suppose to be the sufferings of laboratory animals. It is becoming generally known, however, that investigators customarily use anaesthetics and otherwise treat their subjects with due care; and so, with the spread of popular knowledge, the extreme opponents of vivisection now appear to be losing the influence that not long ago threatened to retard the progress of medical science.

The diseases that are specifically zoological in nature constitute a forbidding menace to human welfare, some of the more important of them being syphilis, yellow fever, African sleeping sickness, dysentery, malaria, and hookworm disease. Each of these is produced by a specific form of animal life (except dysentery, which is produced by several different organisms), living as a parasite in the human body where it multiplies and gives off poisonous matter. The various effects of these animals and their waste materials constitute the symptoms of the diseases for which they are responsible. Healthy individuals become infected by a transfer of the living parasites from a person afflicted with the disease, sometimes directly, by contact, as in syphilis, sometimes through a more or less complicated series of intermediate circumstances, as in malaria and hookworm disease. All of the diseases just mentioned are destructive to a degree which is not commonly realized. For example, Chandler1 states that "of all 1 Chandler, A. C, "Animal Parasites and Human Disease," 3d ed., p. 147, John Wiley & Sons, Inc., New York, 1926.

human diseases there is none which is of more importance in the world to-day than malaria ... It has been estimated to be the direct or indirect cause of over one-half the entire mortality of the human race. Sir Ronald Ross says that in India alone it is officially estimated that malaria kills over one million persons a year." The frightful effects of syphilis are commonly known in some degree, though not fully realized by most people; and hookworm disease has social consequences of tremendous importance.

Parasitism, the living of one organism on or in another, at the latter's expense, is a widespread phenomenon, of which these disease-producing parasites are especially striking examples. They happen to have great medical and popular interest, but the researches carried out in discovering and identifying them, in learning their life histories, and in getting a biological basis for successful treatment, are essentially zoological in character. To illustrate this point, we may look briefly at two of the diseases mentioned above.