The matter of longevity and also of size may also be dependent on the nature of the cells. Minot states that the most characteristic pecu liarity of advancing age, of increasing develop ment, is the growth of protoplasm in proportion to the size of the nucleus. "The possession of a relatively large quantity of protoplasm is a sign of age"; that is, the amount of protoplasm in the cell is large in maturity and old age as compared with the nucleus. He refers to Biit sail's investigations, which indicate that in bacteria a nucleus is present and very large, while the protoplasm enveloping it is minimal in amount. This fact should be correlated with the extraordinary power of proliferation or re production in these organisms, indicating that a small proportion of protoplasm is essential to rapid growth. This presence of a minimum amount of protoplasm is a 'young' character, a characteristic of youth. Certain sea-fishes, as also starfishes, continue to grow for very long periods, and their cells have the 'young' charac ter, containing very little protoplasm. On the other hand, there are certain types which do not grow beyond a definite size, and in these, as for instance among the insects, the cells are charac terized by having a great deal of protoplasm in proportion to the nucleus. In this connection
it should be remembered that May-flies and dragon-flies in the Carboniferous period were gigantic in size, their wings expanding from ten to twenty inches, and throughout geological history we have cases of types becoming colossal before becoming extinct. Thus overweight and oversize may be a sign of weakness and decrepi tude—of senescence of the type. As the result of prolonged observation on guinea-pigs, weigh ing them regularly at fixed ages from birth until maturity, Minot shows that even from the time of birth there is a steady loss of vitality; the animal begins, so to speak, to die as soon as it is born. The amount of protoplasm in the cells increases with age. This loss of growth power is equally demonstrated in the case of man, and presumably of all mammals. The passage from youth to old age is called 'senescence' (q.v.), and the procreation of the young 'rejuve nation' (q.v.).