40 THE DOCTRINE OF DESCENT. 
nation. By this we mean to say that in the province of 
reproduction the limits of inquiry are neither narrow 
nor peculiar. We will therefore now proceed to describe 
the process of reproduction and development in the 
animal kingdom. 
If, as must be generally admitted, the most essential 
characteristics are common _-to the highest and the lowest 
life,—and it is only the complexity of the vital processes, 
together with the variety of the parts by which they are 
performed, that give rise to graduated diversities,—it 
will, of course, be in the simplest organisms that we shall 
most readily recognize the nature of these vital processes. 
The simplest beings, discovered by Haeckel, such as 
the Protamceba, those minute albuminous masses of sar- 
code, increase to a certain extent. Why these dimensions 
should vary only within definite narrow limits, and why, 
on attaining a certain extent, the molecules should 
gravitate into two halves, we do not know; at any rate 
it is an affair of relations of cohesion, theoretically 
susceptible of computation. It is enough that at a 
certain size the coherence of the parts is loosened in a 
central zone, the individual becomes faithless to its 
name, and divides into two halves, of which each from 
the moment of separation begins an individual life, 
while from the commencement of the fission prepara- 
tions were being made for their self-dependence. This 
is the simplest case of reproduction, a multiplication 
by division, Frequently, however, it does not stop 
at bisection; the motion of the minute constituents, 
which causes the fission, proceeds in such a manner that 
the halves are again divided, and the quarters yet again, 
the whole being thus divided into a greater number of 
