I.] THE DURATION OF LIFE. 33 



supposition gains in probability when we are enabled to reduce 

 the Hmitations of the organism in both time and space to one 

 and the same principle. It cannot however be asserted under 

 any circumstances that it is a pure supposition that the ovum 

 possesses a capacity for cell-multiplication which is limited 

 both as to numbers produced and rate of production. The fact 

 that each species maintains an average size is a sufficient proof 

 of the truth of this conclusion. 



Hitherto I have only spoken of animals and have hardly 

 mentioned plants. I should not have been able to consider 

 them at all, had it not happened that a work of Hildebrand's ' 

 has recently appeared, which has, for the first time, provided 

 us w^ith exact observations on the duration of plant-life. 



The chief results obtained by this author agree very well 

 with the view which I have brought before you to-day. Hilde- 

 brand shows that the duration of life in plants also is by no 

 means completely fixed, and that it may be very considerably 

 altered through the agency of the external conditions of life. 

 He shows that, in course of time, and under changed conditions 

 of life, an annual plant may become perennial, or vice versa. 

 The external factors which mfluence the duration of life are 

 here however essentially different, as indeed we expect them 

 to be, when we remember the very different conditions under 

 which the animal and vegetable kingdoms exist. During the 

 life of animals the destruction of mature individuals plays a 

 most important part, but the existence of the mature plant is 

 fairly well secured ; their chief period of destruction is during 

 youth, and this fact has a direct influence upon the degree of 

 fertility, but not upon the duration of life. Climatic considera- 

 tions, especially the periodical changes of summer and winter, 

 or wet and dry seasons, are here of greater importance. 



It must then be admitted that the dependence of the duration 

 of life upon the external conditions of existence is alike com- 

 mon to plants and animals. In both kingdoms the high multi- 

 cellular forms with well-differentiated organs contain the germs 

 of death, while the low unicellular organisms are potentially 

 immortal. Furthermore, an undying succession of reproductive 

 cells is possessed by all the higher forms, although this may be 



See Appendix, note 12, p. 66. 

 D 



