!88 5 .J 
Death and Individuality. 7= 
stitute the cycle. There is much evidence of a positive 
chai after to confirm the belief of the cyclical course of life, 
even among the Protozoa and Protophytes, in which there 
occuis what is known as rejuvenation ( verjiingung ). 
2. I maintain that it is probable that all cycles of cells are 
self-limited. Let us first ascertain the nature of the limita- 
tion. Our knowledge of the manner in which the cycles are 
limited (i.e., of the causes of natural death) is very restrifted, 
and deiived solely from the higher animals. My own special 
investigations have been in this field, and have led me to 
the opinions and problems we are discussing. 
My experiments demonstrate that, when properly analysed, 
the growth of at least the higher animals gradually dimi- 
nishes from birth onwards, almost without interruption. 
Phis is an irrefutable mathematical verification of the views 
which I advanced in my article on “ Growth as a Funftion 
of Cells,” published in 1879, the essence of which, as far as 
we are now concerned, is that the cells of a cycle conti- 
nuously lose their power of division, so that the interval 
between two successive divisions gradually increases. This 
involves the ultimate termination of the cycle, because the 
losses go on, not until the cells can no longer divide, but 
until they exhaust themselves. This whole series of changes 
is properly senescence, or growing old. Senescence is a con- 
tinuous process, covering the whole period of a cycle of 
cells ; and we must assume it is the positive loss of power 
in the single cells, such that the last-produced cells cannot 
continue, and natural death ensues. Of course, in the cases 
of a multicellular animal, death of the whole follows 
secondarily upon exhaustion of any essential part, — as in 
the case of insefts, which die upon laying their eggs. In 
the higher animals, the cycle is limited by senescence, and 
senescence is a decay which probably begins when the cycle 
begins. The next point to decide is, whether the same 
phenomenon occurs with the unicellular organisms. If it is 
found that the divisions of a Paramecium, for instance, after 
a conjugation, are at first rapid, and then follow at increasing 
intervals, it would prove (provided always the external con- 
ditions remained constant) that we here had true senescence, 
with its sequel, natural death, or the end of the cycle. 
Until this point is settled we cannot know whether there is, 
among unicellular animals, a form of death homologous 
with the natural death from senescence in the higher 
animals and plants. 
It is to be regretted that both Weismann and Goette 
appear not to know the article to which reference has just 
been made ; otherwise they would have recognised that the 
