Among the structures of reversionary type that have attracted most attention of late years is the appendix vermiformis. This, as elsewhere shown, is a rudimentary offshoot which is extremely variable. Man retains this structure as a relic of having been at one time a vegetable feeder. In the koala (Australian native bear), a vegetable-feeding marsupial, it is more than thrice the size of the body. In the carnivora it has entirely vanished. In man, where it is sometimes absent and sometimes is as largely developed as in the orang, it is commonly from four to five inches in length and about a third of an inch in diameter. The appendix is poorly supplied with blood, which predisposes it to attacks by microbes because of the absence of leucocytes to fight these, and also because being, so to speak, a blind ally of the intestine, microbes find in it a suitable culture medium for them. The secretions of the appendix are very apt to decompose: hence a culture medium. The extreme variability of this disappearing organ may be judged from the Figs. 108, 109, 110. As it is best developed in degenerates, it constitutes in them one source of predisposition to death from blood poisoning or from sudden shock. The location of this organ also tends to facilitate disease. In degenerates it may be situated at any point upon the end of the big bowel, varying from two to three inches. This little bowel is worse than useless in man, being a source of serious danger. It is an instance of checked development of the same kind which causes the human liver to take on sauropsidian peculiarities. Man in this particular as well as the orang is lower than the carnivora, who have lost this worse than useless organ. Its tendency to disappearance in man indicates once more the truth that degeneracy of an organ is often, through the law of economy of growth, for the benefit of the organism as a whole. I may conclude this outline of human reversionary tendencies by mentioning that merycism, or rumination, has been very frequently found among imbeciles, paranoiacs, hysterics, and epileptics. CHAPTER XVI DEGENERACY OF THE BRAIN