What a wonderful phenomenon is this power in man to reappear in future generations by means of heredity, to transmit his own nature to his descendants by transfusing it--working it into their organism! And no less wonderful is it to see how not only instincts but organs gradually disappear in the course of generations when they are not put into action. In insects, crustaceans, fish, amphibians which have migrated to caverns and have lived for many generations in the dark, the eyes are almost imperceptible, and this is certainly not the result of natural selection, for eyes are not injurious even to beings living in the dark, but solely because, with the cessation of the activity of an organ, it must of necessity retrograde. Three or four generations are necessary before horses completely lose their wild instincts, so that some horse-breeders only choose those that have been already trained in the circus. If one takes two hounds exactly alike (of the same mother and the same litter) and accustoms the one to the chase, the other to watch the house; if one then allows them to breed separately, so as to form two distinct families, one to start the game for man when hunting, the other to guard his house against strangers, we may be certain that after four or five generations their instincts will be profoundly modified. If after ten years one takes a litter from each of these families descended from a common ancestor, and rears them in the same room under the same conditions, far from every noise, and brings them when they are grown into a meadow, it will be seen that, at the report of a gun, the offspring of the dogs trained for the chase will look around as though trying to espy a bird, while the others run off terrified. On the shores of certain almost desert islands birds are found which, like the _Phalaropus_ of Iceland,[38] are very much afraid of man, while those living in the interior of the island are not at all timorous. If one reads Brehm’s 'Animal Life,’ one finds similar instances of fear transmitted from generation to generation, with marked differences in the same species according to the relations which the animals have with man. Although monkeys in general are very timid, and always flee at the sight of man, the _Semnopithecus_ _entellus_, which the Indians worship and honour as a divinity, has become so bold that it enters the gardens, steals everything, plunders the houses, rummages in the trunks and cupboards of the Europeans, and snatches food from off the table or out of their hands. A missionary relates that he was once in a disagreeable predicament, because he had nothing to offer to these impudent monkeys, and that, if he had not defended himself in time with a stick, the animals would have whipped him.[39] [38] Preyer: _Kataplexie_, p. 107. 1878. [39] Brehm: _Säugethiere_. 1883. Vol. i. p. 105.