30 THE DURATION OF LIFE. [I. 



Modern embryology aftbrds us many proofs, in the seg- 

 mentation of the ovum, and in the subsequent developmental 

 changes, that the causes of the difierent forms of reproductive 

 activity witnessed in cells lie in the essential nature of the cells 

 themselves. Why does the segmentation of one half of certain 

 eggs proceed twice as rapidly as that of the other half? why 

 do the cells of the ectoderm divide so much more quickly than 

 those of the endoderm ? Why does not only the rate, but also 

 the number of cells produced (so far as we can follow thcmi, 

 always remain the same? Why does the multiplication of cells 

 in every part of the blastoderm take place with the exact 

 amount of energy and rapidity necessary to produce the 

 various elevations, folds, invaginations, etc., in which the 

 different organs and tissues have their origin, and from which 

 finally the organism itself arises ? There can be no doubt that 

 the causes of all these phenomena lie within the cells them- 

 selves; that in the ovum and the cells which are immediately 

 derived from it, there exists a tendency towards a certain 

 determined (I might almost say specific) mode and energy of 

 cell-multiplication. And why should we regard this inherited 

 tendency as confined to the building up of the embryo ? why 

 should it not also exist in the young, and later in the mature 

 animal? The phenomena of heredity which make their ap- 

 pearance even in old age afford us proofs that a tendency 

 towards a certain mode of cell-multiplication continues to 

 regulate the growth of the organism during the whole of its 

 life. 



The above-mentioned considerations show us that the degree 

 of reproductive activity present in the tissues is regulated by 

 internal causes, while the natural death of an organism is the 

 termination— the hereditary limitation — of the process of cell- 

 division, which began in the segmentation of the ovum. 



Allow me to suggest a further consideration which may be 

 compared with the former. The organism is not only limited 

 in time, but also in space : it not only lives for a limited period, 

 but it can only attain a limited size. Many animals grow to 

 their full size long before their natural end : and although many 

 fishes, reptiles, and lower animals are said to grow during the 

 whole of their life, we do not mean by this that they possess 

 the power of unlimited growth any more than that of unlimited 



