In order to void the contents of the intestines, mammals have to stand still and assume some particular position. Each act of this kind is a definite risk in the struggle for existence. A carnivorous mammal which, in the process of hunting its prey, had to stop from time to time, would be inferior to one which could pursue its course without pausing. So, also, a herbivorous mammal, escaping from an enemy by flight, would have the better chance of surviving the less it was necessary for it to stand still. According to such a view, the extreme development of the large intestine would supply a real want in the struggle for existence. M. Yves Delage,[50] the well-known biologist, is unable to accept this hypothesis. He thinks that the rectal enlargement would fulfil the purpose, and adds that everyone has seen herbivorous animals pass their excretions whilst running. The rectum of mammals, however, cannot serve as a reservoir for waste matter, because as soon as such matter reaches the rectum it excites the need of excretion. The waste matter accumulates in the large intestine, from which it passes into the rectum at intervals. When it has reached that region, a sensation is caused which leads to defæcation. M. Delage is not quite definite when he speaks of mammals voiding their excretions whilst they are in motion. A horse, harnessed to a vehicle, may defæcate whilst it is walking or even running slowly. But these animals cannot defæcate when in rapid motion, and competent observers state that horses never do so whilst racing. In zoological gardens, where animals have room to run about, they stand still before emptying the rectum. M. Ch. Debreuil, who keeps antelopes in a very large park at Melun, has noticed that the excreta are always to be found in masses and not scattered about as if they had been discharged by animals in motion. Antelopes, which are animals that run and leap extremely actively, have to come to a standstill before discharging their small pellets of deer-like excreta. In the struggle for existence, when a mammal is pursuing its prey or escaping its enemy, there is no question of the leisurely movement of a horse harnessed to an omnibus or cab, but the greatest possible activity is necessary. In such circumstances the possession of an organ within which the excreta could accumulate would be of real importance. My theory of the origin of the mammalian large intestine is intrinsically probable. Although the capacity of the large intestine may preserve a mammal in emergencies, it is attended with disadvantages that may shorten the actual duration of life.