The facts which I have been able to get together do not attribute a long life to other running birds. Gurney mentions that a cassowary (_Casuarius westermanni_) lived 26 years in the Zoological Gardens of Rotterdam, and that three Australian emus (_Dromaeus novae-hollandiae_) had lived in the same Gardens for 28, 22, and 20 years. M. Oustalet (_Ornis_, 1899, vol. x, p. 62) mentions another emu of the same species which died in London at the age of over 23 years. The rhea (_Rhea americana_), another large running bird, does not live so long. “Boecking thinks that its duration of life should be set down at from 14 to 15 years. According to him, many of these birds die of old age.” (Brehm, _Oiseaux_, vol. ii, p. 517). It is striking to compare the short life of cursorial birds, which nevertheless thrive and reproduce in captivity, with the remarkable longevity of so many other birds (parrots, birds of prey) which, although they are much smaller, have been kept alive for from 80 to 100 years. It would be difficult to find a more striking argument in favour of the view that richness of the intestinal flora shortens life. When birds become adapted to terrestrial life and acquire a huge large intestine in which microbes can abound, their duration of life is diminished. Just as some birds, losing the aerial mode of life, have come to resemble mammals, so also some mammals have become flying animals, provided with wings and in some respects resembling birds. Bats are the most familiar instance. The large intestine, which is extremely useful to running animals, not only ceases to be an advantage but is harmful to flying creatures, insomuch as it increases the weight of the body uselessly. Bats, accordingly, have no cæcum whilst the large intestine is changed in structure and function. Instead of being a capacious tube, serving as a reservoir for the refuse of the food, the large intestine of bats has the same diameter as the small intestine. Its structure is nearly identical. It is provided with glands, and as I have already mentioned in the last chapter, it digests the food in the same way as the small intestine. In fact, the large intestine has become simply a part of the small intestine, the total length of the gut being reduced. Bats, therefore, can no longer retain their secretions but have to empty the intestine almost as often as most birds. I find that Indian fruit bats (_Pteropus medius_) discharge their excreta very often. Microscopic examination shows that there is an absence of microbes quite unusual in the case of a mammal. The alimentary canal of bats is nearly aseptic, containing only a few single bacteria. I have fed these fruit bats with the same food (carrots) which I have given to rabbits, guinea pigs, and mice; whilst the bats accomplished the process of digestion in 1-1/2 hours, and deposited excreta containing fragments of carrot, the rodents took very much longer for digestion and large quantities of waste matter