For the psychologist working with human subjects, the relation of parental age to offspring behavior has long been a matter of great interest. One need only examine the literature of the clinician to appreciate the complexity of the problem. The relationship of the woman of 40 years of age to her new-born infant and that of the developing child to the mother is frequently marked by traumatic incidents which affect the behavior and the development of the young organism. Even growth and physiological health are impaired when the normal expressions of love and affection are inhibited by repression and distortion accompanying the feelings of age differences. In the field of comparative and animal psychology, however, the experimental implications of age relationships have been largely neglected. Few investigations have dealt with the topic of this monograph for reasons which will be sketched briefly in subsequent discussion. Before describing what has been accomplished in research on age in the field of animal behavior, it is advisable to introduce some methodological considerations which affect experimental productivity in this area. Age has been a relatively ignored parameter in psychological investigation except in so far as it is restricted to the so-called formative period of infancy and childhood. Much attention has been paid to the effect of early experience upon later adult behavior. Scott’ and the other participants in a conference on this topic have presented an inclusive research program on both the human and animal levels which repays reading by all interested in comparative research on age factors. This emphasis has been considerably influenced by Freudian hypothesizing and by the need to seek experimental validation for the concept of childhood’s importance. Significant research has pointed to the necessity for adequate longitudinal studies in which the origin of adult behavior patterns can be seen in early infancy. Papers by Schneirla6 and Riess3 at a symposium on field methodology in comparative psychology document this point of view. Other studiese have shown that there are critical periods and unique types of learning in infancy which throw Iight upon adult behavior. The development of the organism’s actions, like that of its morphology, does not appear to occur in a linear function. Smaller quantitative variations seem to accumulate and to result in relatively sharp, qualitative and, frequently, gross modifications of behavior. This finding has been energetically dealt with in many researches. Hebb,2 in his fascinating and important book on the development of behavior in organisms, initiated the concept of early and late learning as determining differential adult types of adaptation. Scott6 and his co-workers at Bar Harbor have done much stimulating research on the appearance and meaning of “critiral” periods in the maturation of puppies and other animals.