Thus it would seem probable that, contrary to a belief once widely prevalent, the sexual instinct has increased rather than diminished with the growth of civilization. This fact was clear to the insight of Lucretius, though it has often been lost sight of since.[225] Yet even observation of animals might have suggested the real bearing of the facts. The higher breeds of cattle, it is said, require the male more often than the inferior breeds.[226] Thorough-bred horses soon reach sexual maturity, and I understand that since pains have been taken to improve cart-horses the sexual instincts of the mares have become less trustworthy. There is certainly no doubt that in our domestic animals generally, which live under what may be called civilized conditions, the sexual system and the sexual needs are more developed than in the wild species most closely related to them.[227] All observers seem to agree on this point, and it is sufficient to refer to the excellent summary of the question furnished by Heape in the study of "The 'Sexual Season' of Mammals," to which reference has already been made. He remarks, moreover, that, "while the sexual activity of domestic animals and of wild animals in captivity may be more frequently exhibited, it is not so violent as is shown by animals in the wild state."[228] So that, it would seem, the greater periodicity of the instinct in the wild state, alike in animals and in man, is associated with greater violence of the manifestations when they do appear. Certain rodents, such as the rat and the mouse, are well known to possess both great reproductive power and marked sexual proclivities. Heape suggests that this also is "due to the advantages derived from their intimate relations with the luxuries of civilization." Heape recognizes that, as regards reproductive power, the same development may be traced in man: "It would seem highly probable that the reproductive power of man has increased with civilization, precisely as it may be increased in the lower animals by domestication; that the effect of a regular supply of good food, together with all the other stimulating factors available and exercised in modern civilized communities, has resulted in such great activity of the generative organs, and so great an increase in the supply of the reproductive elements, that conception in the healthy human female may be said to be possible almost at any time during the reproductive period."