398 THE AMERICAN JOURNAL OF SOCIOLOGY
likewise to the dimensions of organisms, but under special con- ditions.
In individual organisms the size of the structure depends principally upon heredity. The quantity and quality of the alimentation occasion only slight variations in the size. The individual dimensions are above all fixed from the beginning. From the egg whence the individual develops itself they are fixed, circumscribed by the numeric and chronological limits it the rapidity of formation of the cells.
Why, now, are these divers limits in the multiplication of the cells variable according to the organisms? Here again we can refer to the law of proportionality of the surface to the mass, and to the physiological conditions of opportunity. Says Weis- mann :
In fact, when a particular size is more favorable to the development of a certain plan of construction, there has resulted a process of selection whicl has brought the fixation for each species to an average size, oscillating within more or less extended limits. This average size is transmitted by heredity from generation to generation, and the model is already contained in the germ of each individual. 1
It is the same with the duration of organisms. Weismann shows us clearly that natural death is only a phenomenon of adaptation useful to polycellular organisms, and therefore, on the contrary, that simple cells may be immortal. The germinative cells (such as eggs and spermatozoids) which, differentiated in the multi- cellular organisms from the somatic cells, alone conserve immor- tality. For the unicellular animals there can be no question oi natural death. Their increase is produced by division. The new cells are identical with the old ones. Among these animals there are neither older or younger beings. The myriads of cells are all equally old and equally young, like their species. Their life is indefinitely extended into the past and future througl incessant divisions.
The limitation of the individual by death is therefore not a constant law inherent in the very essence of life. It is only a necessary adaptation, and this is realized only in the superioi
1 Limites de la croissance et de la duree de la vie.
