It is easy to explain that human races so different could develop separately in continents and under climates with a very different mode of life and conditions of development, if we reflect that at these remote periods men only had very limited modes of transport and lived in a fashion very little different from that of the anthropoid apes, so that the ethnological forms were preserved separated from each other by small distances. This fact can still be observed among the small hostile Indian or Malay tribes, who live in tropical regions and often occupy only a few square leagues. The higher civilizations of former times could not develop beyond a comparatively limited circle, as their means of transport did not allow them to venture too far. The conquest of the whole earth by modern civilization by means of the mariner's compass, firearms, steam and electricity is thus an absolutely contemporaneous event, unique in the history of the world, the origin of which hardly goes back more than four hundred years. This event has completely upset the natural internal evolution of human races, by the fact that all the lower races attacked by civilized races armed with guns and alcohol, are destined to rapid and complete destruction. Geology has discovered in the caves of the quaternary period, human remains which are much lower in the scale of evolution and much nearer the anthropoid apes than the lowest races still living. Their brain, as shown by the cranial cavity, was still smaller. Lastly, _Dubois_ has discovered in Java the cranium of _Pithecanthropus erectus_ which is intermediate between that of the orang-utan and man. If more such remains are discovered the chain of transition between the apes and man will be almost complete. =Hybridity. Consanguinity.=--Before concluding this chapter we must study the question of _hybrids_. It is important to know to what point fecundity and descent are influenced by the degree of relationship between the two procreators. Conjugation probably arises from the general necessity of organisms to reënforce their race by variety. Consanguinity perpetuated is harmful to the species, in the same way as parthenogenesis, or indefinite reproduction by fission or budding. It produces enfeeblement and degeneration of the race, and leads to extinction by causing sterility. By _consanguinity_ is meant continued sexual union between near relatives. It is easy to understand that the conjugation of two germs derived from brothers and sisters or from a father and his daughter approaches parthenogenesis from the point of view of the mixing of hereditary energies. We shall see later on that nearly all peoples have a certain repugnance to consanguineous marriages. Among animals, natural selection eliminates too consanguineous products.