No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 95 
The structure of a fully developed adult animal, it seems to 
me, must depend on the relation that exists between (1) the 
rate of production of new cells, (2) the rate and the amount of 
specialization of these cells, (3) the longevity of the completely 
specialized cells, and (4) the death rate of the cells, or their 
rate of decline toward a simpler, less specialized condition. 
The interrelations of these factors must be extremely complex 
and to a certain extent independent of each other ; for repro- 
duction, specialization, and decay may, apparently, take place 
simultaneously in any part of any organ, or in the entire 
organism, in almost any conceivable proportion. 
An essential feature of organic death in the higher animals 
is the cessation of normal activity in many different organs, 
because other organs, by accident, or by inherent conditions, 
cease to perform some particular work, perhaps insignificant 
in itself, upon which all the others depend. 
The nerve cells, for example, may be performing faithfully 
and well their particular work, and yet may perish for lack 
of proper nutrition, or owing to the presence of poisonous 
substances that should have been eliminated from the body. 
We do not know how long they might have continued to act 
under favorable conditions. 
Every living thing has a more or less definite size, form, 
life period, and kind of activity. It is apparently assumed by 
some writers that these manifestations are not merely pre- 
determined by the organization of the ovum, acting under the 
guidance of a changing environment, but that there is an actual 
deposit of formed material corresponding in some way to every 
one of the almost infinitely numerous parts of the future 
organism. That there are definite potentialities in every ovum 
cannot be denied, but these potentialities must not be con- 
founded with what actually exists as specific matter, and which 
forms the actual physical basis of the ovum at a given time. 
The future organism is the resultant of the action on an 
initial, specifically constructed mass of forces outside of and 
independent of the mass (1) of such forces acting on the newly 
formed material added to the old, and (2) on the continued 
interaction of these masses and forces on one another, under 
