The significance of these differences depends largely on whether they are germinal, or merely the results of social tradition. In favor of the view that they are in large part racial and hereditary, is the fact that they persist in all environments. They are found, as Professor Mecklin says, "Only at the lower level of instinct, impulse and temperament, and do not, therefore, admit of clear definition because they are overlaid in the case of every individual with a mental superstructure gotten from the social heritage which may vary widely in the case of members of the same race. That they do persist, however, is evidenced in the case of the Negroes subjected to the very different types of civilization in Haiti, Santo Domingo, the United States, and Jamaica. In each of these cases a complete break has been made with the social traditions of Africa and different civilizations have been substituted, and yet in temperament and character the Negro in all these countries is essentially the same. The so-called 'reversion to type' often pointed out in the Negro is in reality but the recrudescence of fundamental, unchanged race traits upon the partial breakdown of the social heritage or the Negro's failure successfully to appropriate it." Again, as Professor Ferguson points out, the experimental tests above cited may be thought to give some support to the idea that the emotional characteristics of the Negro are really inherent. "Strong and changing emotions, an improvident character and a tendency to immoral conduct are not unallied," he explains; "They are all rooted in uncontrolled impulse. And a factor which may tend to produce all three is a deficient development of the more purely intellectual capacities. Where the implications of the ideas are not apprehended, where thought is not lively and fertile, where meanings and consequences are not grasped, the need for the control of impulse will not be felt. And the demonstrable deficiency of the Negro in intellectual traits may involve the dynamic deficiencies which common opinion claims to exist." There are other racial and heritable differences of much importance, which are given too little recognition--namely, the differences of disease resistance. Here one can speak unhesitatingly of a real inferiority in respect to the environment of North America. As was pointed out in the chapter on Natural Selection, the Negro has been subjected to lethal selection for centuries by the Negro diseases, the diseases of tropical Africa, of which malaria and yellow fever are the most conspicuous examples. The Negro is strongly resistant to these and can live where the white man dies. The white man, on the other hand, has his own diseases, of which tuberculosis is an excellent example. Compared with the Negro, he is relatively resistant to phthisis and will survive where the Negro dies.