No fact of mind is valueless for genetic psychology. There are, however, four major groups of psychological materials which are contributing heavily to the advance ment of genetic description. They are (1) the facts of individual development as gained from studies of foetal or embryonic life, infancy, childhood, adolescence, maturity and senility. (2) Of mental characteristics of organic types or races, including the study of man, other animals and plants. For there is as much reason to speak of the psychology of the rac coon, fox, dog, horse, monkey, ape, as of that of man. Indeed, the investigation of more nar rowly limited groups of organisms follows hard upon the description of a particular type of living thing. Thus, in ethnic psychology, the attempt is made to describe various kinds of human mind. Similarly, we might have, and doubtless shall have, in the course of the de velopment of science, canine, bovine, feline, piscian and simian ethnic psychology, just as we now have the human. (3) Facts of social relations, which constitute the psychology of social groups. These must necessarily be used because it is the rule that individuals exist in social relations and are mentally different be cause of the social features of their environ ment. The evolution of social relations, social groups, institutions, is one phase or aspect of the great task of genetic psychology. (4) The facts of mental deficiency, deviation and dis ease. These should be made to contribute to genetic psychology, because in psychopathology, stages, phases and peculiar complexes of mind appear dissociated, exaggerated, as it were held up for inspection. In the insane man or lower animal, certain phenomena of response which under normal conditions are difficult to observe may appear clearly. The genetic psychologist would be stupid indeed-were he not willing and eager to make use of these apparently bizarre but actually extremely illuminating conditions of mental life in the interests of genetic de scription.
Genetic psychology, a historical science, lacks historical background, for as a special aspect of psychology, it has only re cently been recognized. Formerly, it was hid den in comparative, animal and child psychology.
Clearly enough, the ground for its develop ment was prepared by the evolutional biologists in the last century, notably by Darwin, Wal lace, Huxley, Lamarck and Weismann. These and many other observers prepared the way for our modern inquiries into the history of mind. Darwin himself contributed important material to genetic description through his studies of various animals; and somewhat later Romanes set himself the special task of outlining the course of mental evolution.
From these modest beginnings in England, interest in what only very recently has come to be known as genetic psychology spread over the world. The recent developments of this division of psychology can best be described in connection with methods and biological relations.
Systematic comparison of facts is the natural and necessary procedure of the genetic psychologist, since he needs above all knowledge of the developmental or genetic re lations of his materials. It is not strange, then,
that comparative psychology should be used as almost synonymous with genetic psychology. That this usage is not scientifically satisfactory becomes evident when we realize that in strict ness comparative psychology should designate a special method of inquiry, whereas genetic psychology should designate a special purpose, task or scientific goal, namely, the description of mental development and evolution.
The closest of relations exists between ex perimental biology and genetic psychology, for many of the chief problems of the latter are those of heredity. It is not surprising, thete fore, that the development of a special science of genetics (for the study of the phenomena of heredity, see GENETICS) should have occurred in biology simultaneously with the development of genetic description in psychology. Ideally, genetics as a biological science should include the study of mind. But since most biologists do not so understand the term, it is doubtless desirable to continue to designate the study of mental development in evolution as genetic psychology or psychogenetics.
Further, the genetic division of psychology is dependent upon other and varied biological sciences, in which ordinarily a student of hu man psychology has slight professional interest. Among these are palmontology, anthropology, experimenttl zoology, comparative physiology, comparative neurology and neuropathology. For from these special sciences, facts pertinent to those of mental evolution may be derived by the skilled observer. Indeed, everything having to do with life sooner or later becomes of service to the genetic psychologist. An emi nent student of human psychology once re marked, "I am interested in everything human? He referred to the products of human activity as well as to the facts of mind. The genetic psychologist must be able to say, aI am inter ested in everything living.* There are two slightly separable classes of problems in genetic psychology, ques tions of origin and, questions of development or genetic relationship. The former are diffi cultly soluble; the latter are much more amen able to study.
Of the problems of genetic relations, upon which attention is focused, the following are some of the most important: (1) Problems of receptivity or the develop ment of sensibility in its various modes, quali ties and relations. These all have to do with the so-called special senses or their forerunners.
(2) Problems of affectivity. The appear ance in the individual or in the race of forms of feeling, emotion, sentiment; their character istics, their relationships and their modes of expression are investigated. It has been sug gested that feeling, or affectivity, as a mental factor in life, may antedate sensation. This, however, is a problem of origins which re mains unsolved. The growth of the affective life in man from birth to maturity is one of the most fascinating chapters of psychology.