In man there is no sharp limit between youth and maturity unless it be marked by puberty, but in lower forms of life it is demarcated by a metamorphosis. The tadpole, for example, becomes a frog in the third or fourth month of its life and this process can be accelerated by feeding the creature with thyroid, no matter from what animal. Gudernatsch was able to make frogs no larger than a fly. Allen showed that the tadpole with the thyroid removed can never become a frog, although it may live long and continue to grow larger than the usual tadpole; but if such aged tadpoles are fed with thyroid they promptly become frogs. Salamanders metamorphose by merely throwing off the gills and changing the skin and tail, and the Mexican axoloti maintains the tadpole form through life; but even it, when fed with thyroid, promptly metamorphoses. Schwingle induced metamorphosis in tadpoles by feeding them with a trace of inorganic iodine. Thus the duration of the tadpole stage seems to be the time required to secure a certain compound containing iodine. Insects hatched as maggots will become chrysalides and then flies, but if thyroid is fed to the maggot it accelerates the metamorphosis, although we do not know whether it is due to the accumulation or formation of definite compounds. Loeb sought to determine whether the duration of the maggot in the larval stage could be due to temperature and he found that this had effects similar to those described above. The larval period lasted 5.8 days at 25° C. and 17.8 days at 15°. The total duration of life was 38.5 days at 25° and 123.96 at 15°, both ratios being 1 to 3. Thus the influence of temperature upon the larval period was like that which it exerted on adult life. The same effect he found in salamanders, all of which suggested to him the conclusion that the duration of life and of the larval period is really the time required for the completion of certain chemical reactions. The cessation of respiration, which means death, and alterations in circulation, which mean metamorphosis or the death of youth, are critical periods and perhaps both points are reached when a certain toxin is formed in sufficient quantity or when a necessary substance is destroyed or reduced. Thus a shortened youth can, in amphibians, be prolonged by modifying the temperature or offering the specific substance that causes metamorphosis, namely, iodine or thyroid. There is no end to the substances capable of hastening death; shall we ever find one that can prolong life?[176]