But this interpretation is false to Darwin’s own principles. While Darwin did describe and lay great emphasis upon the tooth and fang struggle for existence, he noted and stressed the fact that even among animals, modifying influences were at work. He made clear that co-operation exists among many species of animal life, and that this co-operative tendency is an important survival factor. He also saw that among the highest types of animals there were new and complex expressions of co-operation, and that the higher mental activity of these animal types seemed to be a correlate in some way of the greater co-operative spirit. The application of this principle to human progress implies that the co-operative spirit may ultimately become the chief survival force, and that some day the “fittest” to survive will be those individuals or groups of individuals who co-operate most wisely. This theory will be developed further in the chapter upon “Co-operation Theories in Sociology.” The chief contributors have been Kropotkin and Novicow. Darwin made another important contribution to social science in his theory of sexual selection. This idea is a phase of natural selection. Among the higher animals the females choose their mates. The males, for example, with the singing voice and beautiful plumage, are the most likely to be chosen. These males thus become the progenitors of the next generation of the given species; the less attractive males mate if at all with the inferior types of females. Thus signs of male attractiveness come to possess survival value.[XVI-1] Among human beings the principle of sexual selection operates, but in a reversed sexual form. During the earlier centuries of human history the custom developed whereby the males took the initiative in choosing mates. As a result, the females resorted to all sorts of devices to make themselves “attractive” and to get themselves “selected.” The social theories of Herbert Spencer (1820–1903) have caused more controversy than those of any other writer in the sociological field. The fact that in these controversies the ideas of Spencer have usually been worsted will not blind the fair-minded seeker after truth to the important rôle which Spencer took in the field of social thought. Spencer early developed the habit of causal thinking, that is, he believed in causes, and hence searched everywhere for causes. Because of the acrimonious discussions which took place between his father and mother, and because of his own independent nature, he repudiated the orthodox religious explanations of the universe. He was trained for the profession of civil engineering. His studies in mathematics and mechanics accentuated his precise and somewhat materialistic interpretation of the universe. His social theories are an outgrowth in part of his emphasis upon the laws of co-existence and sequences in the physical world.