140 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
limited time. Later on, among the higher metazoa, the somatic 
cell became endowed with the power of prolonging its life for 
several generations, and life became correspondingly longer. This 
formation went hand in hand with the differentiation of the cells 
of the organisms, according to the principle of division of labour 
in cells of generation, and was brought about by the process of 
natural selection. This biogenetic principle applies only to the 
many-celled beings ; it has no application to the one-celled beings. 
Life is a lasting one, not a periodically interrupted one. Since it 
in its lowest forms has appeared on the earth, it has continued 
without interruption, only its forms have changed; and all 
individuals of all forms which live in the present day, even of 
the highest, have been derived in an uninterrupted chain from 
that first and lowest. There exists then a complete continuity 
of life. Death is not at all that limitation of life which has 
been assumed to be a necessary attribute of all organisms, but 
does not apply to monads or one-celled beings, including higher 
organised infusoria, as well as the amoebae. The argument for 
this rests, Weismann says, on the one hand, on the reproduction 
by scission or division in the monoplastic cells; and, on the 
other hand, through the necessity of reproduction of the retention 
of the one -celled condition of development among the poly- 
plastides or many-celled beings. 
As death itself, so also the shorter or longer duration of life 
rests entirely on fitness. Death does not rest in a primordial 
quality of the living substance, nor is it necessarily united with 
reproduction, nor even a necessary consequence of it. 
In the observations I have just been making I have been quoting 
mainly from Weismann. The language is, I am afraid, somewhat 
involved, and perhaps has suffered in translation ; but what it 
appears to come to is this, that monads, by the very fact of their 
reproduction being by division, do not die a natural death. 
The one becomes two separate beings. The two again subdivide 
and so on ad infinitum. Possibly this is something of a truism, 
but I do not think that anyone before Weismann has elaborated 
the matter. 
The practical applications of the knowledge of the life history 
of the various forms of microbes are numerous. I have referred 
incidentally to some. Even the potato disease, which no doubt 
is due to the development of air-borne spores, where a suitable 
