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THE AMERICAN NATURALIST [Vol. XLI 



crossing of some tall and dwarf plants produces those of intermediate 

 height. The omission of such limitations may cause a student to 

 believe that Mendelism is the universal law of inheritance. 



F. T. L. 



The Problem of Age, Growth, and Death.— In a series of six 

 public lectures Professor Charles S. Minot has made known the results 

 of his studies, now in progress, concerning the essential nature of 

 senescence. Rejecting such criteria of old age as a halting gait or 

 arterio-sclerosis, which pertain chiefly to man, he has sought those 

 features which apply as well to the aged frog or fish, and even to still 

 lower forms. Such characteristics are found in the decreasing rate 

 of cell division, the increase of protoplasm at the expense of the 

 nucleus, and the progressive differentiation of the protoplasm. Old 

 age is therefore essentially a cytomorphic phase. 



The rate of cell division is expressed by the "mitotic index" which 

 is the average number of mitotic figures found, in sections, among a 

 thousand nuclei. The mitotic index falls from 18 to 13 in rabbit 

 embryos of 7^ and 13 days respectively. Drawings, on the same scale, 

 of nuclei of the various tissues in rabbit embryos of 7^ to 16^ days 

 show a striking reduction in the actual size of the nucleus, except in 

 the case of the nervous tissue. Even there, in relation to the proto- 

 plasmic mass, the nucleus may be relatively small. 



The rate of growth begins to decline before birth, and this rate of 

 decline rapidly decreases until old age, when growth is at its niiiiimuni. 



held to apply to man, both in physical development as shown by 

 statistics of weight, and in mental development as determined by 

 psychologists. During the first months after birth, progress in 

 acquiring concepts of time, space, the ego, and the external world is 

 more rapid than in later years. As with weight, the rate of decline 

 is most abrupt at the outset, becoming gradual as age advances. 



The study leads to the paradoxical conclusion that the changes of 

 senescence are most marked in the years of infancy, for the popular 

 idea of maximum efficiency as the mark of maturity is set aside. 

 The embryo in adding an ounce to its wcioht is rated as advancing 

 more rapidly than the child in gaining a poumi ; the inx . t which leaps 

 many times its own length would be regarded as more successful in 

 jumping than the mammal which can far outdistance it. 



