These facts relating to growth are of fundamental importance for a correct interpretation of the oft-discussed phenomena of early arrest of growth. We have seen that among members of the same race a prolonged period of growth goes hand in hand with unfavorable development, while an abbreviated period of growth results in larger dimensions of all physical measurements, and in a superiority of mental activity. In this statement pathological cases of complete premature arrest of development, or of over-development, are of course excluded,—cases of dwarfish growth or of microcephaly, as well as cases of hypertrophic growth of organs. It follows, that, in judging the physiological value of arrest of growth, the mere fact that growth ceases in one race at an earlier time than in another cannot be considered as significant in itself without observations on the rapidity of growth. So far, the question still remains open, in how far there may be changes in the types of man that cannot be explained by acceleration or retardation of growth. An attempt has been made by Rieger to explain differences in head-form as due to the effect of physiological and mechanical conditions, and Engel emphasizes the effect of pressure of the muscles upon the forms of the head. Walcher tries to explain different head-forms by the consideration of the position of the infant in the cradle. He believes that position on the back produces round heads; position on the side, long heads. It would seem, however, that the difference of head-form in large areas of Europe, in which infants are treated in the same manner, are too great to make this explanation acceptable. A number of observations have been made, however, which demonstrate conclusively a difference between urban and rural types. These observations were first made by Ammon, who showed that the urban population in Baden differs from the rural population in head-form, stature, and pigmentation. He accepts the conclusion that we have here an actual change in type; due, however, not to a direct effect of environment, but rather to an elimination of certain types in city life: in other words, an effect of natural selection. This observation is in accord with observations made by Livi in the cities of Italy, which show also a difference when compared to the surrounding country. Comparisons of the normal and hospital populations of London, made by Shrubsall, are not unfavorable to the assumption of a certain amount of correlation between morbidity and physical type, although the homogeneity of material from a metropolis like London, drawn from different social strata of a large city, remains always open to doubt.