98 PATTEN. [Vou. XII. 
cells is cut down more and more, till only the simplest kinds 
remain, or because the new ones die before they become 
specialized. The lack of specialization may affect different 
parts unequally, as when the surface details of structure fail 
to appear on the otherwise well-developed appendages, as in 
Fig. 21, or the nerve-cords, the appendages, or sense-organs 
are lacking. 
As we might expect, the process is never exactly the 
same, but it invariably tends to carry the organism back, in 
the main, over the old lines of progressive development, till 
it reaches its primitive condition, namely, a small community 
of untrained, insubordinate individuals, which in turn die 
one by one, till the death of the last survivor exterminates 
the race. 
This may be called the true natural death of an organism,— all 
others are more or less catastrophic, due to the increasing lack 
of codrdination and adjustment to the new conditions produced 
by itsown growth and specialization. A natural death like this 
is only possible where the degree of specialization has been com- 
paratively small, and where every cell may receive the stimulus 
and material necessary to the continuance of its activities and 
the discharge of its waste or noxious products. 
If this be true, then there is no such distinction to be made 
as “mortal and immortal” protoplasm. All protoplasm is “ im- 
mortal,” in the same sense that chemical compounds, or mix- 
tures of the same will continue to be formed, manifest their 
specific properties, and disappear so long as the proper environ- 
ment is maintained. 
Cessation of vital activity, then, is due solely to inadequate 
environment, whether we are dealing with highly organized 
individuals, or with bacteria, amoebae, or human ganglion cells, 
or with any part of these. 
The growth of the smallest protoplasmic part of a cell differs 
from the growth of a crystal in the fact that, having grown, the 
conditions of growth and persistency are more materially 
altered in the former than in the later case. 
The metabolic changes that take place in and around the cell 
are to a certain extent processes of filtration. The vital pro- 
