As to animal longevity, no doubt there are real ephemerids. Life can also be prolonged by desiccation or by freezing. Certain it is that many species do not live to see their offspring. In many of the lower forms of life the larval is far longer than the adult stage. The seventeen-year locust, for example, lives out most of its time underground, the imago form continuing but little more than a month. Most butterflies are annual, although those that fail to copulate may hibernate and live through another season, while some are known to have lived several years. Worker bees do not survive the season but queens live from two to five years. J. H. Gurney[29] thinks the passerines are the shortest-lived birds, averaging from eight to nine years, that the lark, canary, bullfinch, gull, may live forty years, the goose fifty, and the parrot sixty. To the latter bird a mythical longevity is often assigned, one being said to have spoken a language that had become extinct. As to animals, domestication prolongs while captivity shortens their normal length of life. Longevity is often related to fertility. Beasts of prey breed slowly and live to be old, while the fecund rabbit is short-lived. But for increased fecundity species subject to high risk would die out. While there is a certain correlation between size and age, since large animals require more time to grow, it is extremely limited. Bunge thinks that in mammals the period the new-born take to double in size is an index of the normal duration of life; but this, too, has its limitations. Some stress diet as an essential factor and others think that length of life may be inversely as the reproductive tax levied upon the system. But of all these questions our knowledge is still very limited.