The author sketches the differences between Homo primigenius (neandertalensis) and Homo sapiens and, on the ground of the main advance consisting in greater skull capacity and brain development, concludes that we must seek the effective cause of the change from the one species to the other in the action of the libido, which, working through primitive repression, introduced the component sexual impulses into the life of the mind and thus replaced the organic factors, of whatever nature, which in animals sufficed to give rise to new species, by a mental factor which brought about a parallel result in developing man. The use of tools, i.e. the projection of bodily organs on to the outer world (Kapp), is the result and not the cause of the evolution of man, these means of culture corresponding to the return of the repressed, not as bodily organs, but in the form of artificial tools.