Climate has a powerful action on the types of races, the latter being generally adapted to the climate in which they live. Thus, the European becomes darker in the tropics while negroes and Indians become paler in the north. LAWS OF RESEMBLANCE--HYBRIDS Every animal species has an instinctive repugnance to pair with another. Even where they are possible, natural hybrids are rare, and only become a little more frequent in domestic animals and plants. The fecundity of hybrids diminishes when they have connection among themselves, and this explains why the instinct for such connections tends to gradually disappear. In his book on "The Mneme," Semon explains the infecundity of hybrids in a very plausible manner, by the disorder that a too large quantity of dissimilar hereditary engrams causes in the hereditary mneme of two conjugated cells. When the parents differ from each other only in a moderate degree homophony may still be reëstablished, and then the divergencies have a very favorable effect on the product, by the new combinations which they furnish in the course of its development. Moral ideas follow the course of instincts, and this explains why sexual connection with animals is regarded as a horrible crime. This is especially produced by pathological aberration, or when one sex is completely isolated from the other. There is also a certain degree of aversion to copulation between different races, in animals as well as man; for example, between sheep and horses of different races, and between white men, negroes and Indians. There are, however, many hybrids or half-breeds in South America, and in Mexico they even constitute two-thirds of the population. Broca maintained that human hybrids produced by the crossing of remote races, for example, between English and negroes or Australians, were usually sterile. Westermark disputes this, but agrees that these hybrids become enfeebled in a few generations. It has also been established that mixed marriage between Jews and Aryans are generally less fecund; but this fact is not yet sufficiently explained. Mulattoes, or hybrids between negroes and whites, constitute a degenerate race and hardly viable, at any rate if their descendants do not return entirely to one of the original races. Half-breeds between whites and American Indians, also called Ladinos, seem on the contrary to form a viable race, but one of little valor. PROHIBITION OF CONSANGUINEOUS MARRIAGES Sexual union between near relations nearly always causes a feeling of repugnance in man, and has been stigmatized by the term _incest_. Coitus between mother and son especially excites disgust. Sexual connection between parents and children, as well as between brothers and sisters is, however, common among certain tribes. Many other races allow marriage between brothers and sisters, but this is elsewhere generally condemned.