However, the numbers and percentages given should be read to indicate the probabilities and should not be interpreted as implying the mortality relationships since different varieties have differing powers of resistance to the same pathological state. The margin of safety in any given group for one or several different disease entities cannot at present be stated with any degree of precision but this factor is doubtless very great. The work of physiologists suggests that there is a reserve power in the human lung sufficient to sustain life until five- sixths of the functionating organ is useless, and I shall quote a case of an opossum wherein only one-tenth of the respirable surface seemed to have remained; we have repeatedly seen both lungs of a monkey apparently entirely solid. Such physical vital incompatibilities might be exemplified by many other cases, but when one reviews the physiological margin of safety, inexplicable and contradictory instances are equally numerous. I have seen a male deer run a doe against the fence and butt her, without result, whereas in an apparently similar occurrence the animal would be dead in a short time. Numerous instances of slight enteritis of a short stretch of duodenum or ileum have killed, with almost nothing to be found microscopically, and on many occasions we have been chagrined in being unable to discover the cause of death. The capacity of self-healing is a variable one, but seems in direct proportion to the quietness and seclusion possible for the animal and inversely to the chance of bacterial infection. The effect of captivity has been the subject of much speculation. For the preservation of health it would seem that animals require periods of rest and activity, thorough elimination, possibly a moderate exercise of their procreative functions, but most of all, appropriate food obtained by the physical effort we term chase. All but the very last condition is supplied in a measure in well managed collections. The degenerating effect of the absence of chase must be admitted. An interesting and suggestive example of this was noted by Mr. Jones at the London Zoological Gardens. He observed the skull of a lion that had been in captivity thirteen years, in which the canine area of the face and the part of the skull acting as the insertion for the seizing and holding muscles had undergone atrophy while the chewing muscles with their bony bases had remained normal. Numerous examples of disease atrophy are on record and those of a physical nature must have counterparts in the realm of physiology. The size to which an animal will attain cannot be estimated by the examples seen in menageries. Judging by the accounts of collectors and hunters and upon the more reliable of the moving picture displays of wild animals in their native haunts, it would seem probable that under normal conditions of habitat the average size of wild beasts is considerably in excess of that in park specimens.