We need not pursue this subject further. It will suffice to offer the general conclusion that the negroid race, while, through its change of habits from the hunting to the agricultural status, it has made an advance both mentally and physically beyond the Pygmy aborigines, does not appear to have advanced greatly in either particular, the negro reaching a mental limit at a low level, and being arrested physically while still possessing marked characteristics of the man-ape. For the higher development of man, under the stress of a more energetic conflict with the conditions of nature, we must seek the continent of Europe, whose human inhabitants had not only to subdue the wild beasts and teach the earth to bring forth wholesome food in place of useless plants, but also to battle with wintry climates, and overcome the adverse influences of cold, sterility of soil, and other hostile conditions of the northern zones. One of the chief problems of biology has long been that of the production of new varieties and species of animals as an effect of gradual variation in structure. This is believed to be ordinarily due to changes in the conditions of nature, animals and plants which have made accordant changes in structure being preserved, those which have not changed in accordance with the new conditions perishing. Where the conditions of nature remain uniform, species may persist for long ages unchanged, though even in the latter case changes in structure are apt to occur, since variation in species is not wholly dependent upon external changes. To a considerable extent it is due to causes existing within the organism itself, fortuitous variations being occasionally preserved when not out of harmony with the state of affairs prevailing in the external world. Or variation may occur through the establishment of new relations between the species inhabiting some locality while inanimate nature remains uniform, or through migration into new inanimate or animate surroundings. Variations, in short, may arise under the influence of any change in the general environment which renders necessary adaptive changes in structure. But this adaptation in some cases takes place in the mind, new actions or methods of meeting the contingency being adopted which render physical changes unnecessary. The problem is a highly complicated one, and no doubt many causes have to do with the multiplicity of effects. There have very likely been many occasions where the changes in structure took place rapidly, in consequence of sudden variations in natural conditions. Such rapid changes in conditions necessarily exert a severe stress or strain on organisms, either destroying them or causing an equally rapid adaptation, physical or mental. In such instances it is likely that many species perish, the change demanded being too great; others escape by migration to better fitted localities; and others, more mobile or less affected by the change, survive through adaptive variations.