that is, it reacts to stimuli too feeble to be regarded as the cause
of its reaction. It engulfs microscopic plants, and digests them in
the internal protoplasm by the aid of an acid secretion. It breathes
oxygen, and excretes carbonic acid and urea, through its whole body
surface. Its mode of gaining the energy which it manifests is
therefore apparently like our own, by combustion of food material.

[Illustration: 1. AMOEBA PROTEUS. HERTWIG, FROM LEIDY.
_ek_, ectosarc; _en_, endosarc; _N_, food particles;
_n_, nucleus; _cv_, contractile vesicle.]

It grows and reaches a certain size, then constricts itself in the
middle and divides into two. The old amoeba has divided into two
young ones, and there is no parent left to die, and death, except by
violence, does not occur. But this absence of death in other rather
distant relatives of the amoeba, and probably in the amoeba
itself, holds true only provided that, after a series of
self-divisions, reproduction takes place after another mode. Two
rather small and weak individuals fuse together in one animal of
renewed vigor, which soon divides into two larger and stronger
descendants. We have here evidently a process corresponding to the
fertilization of the egg in higher animals; yet there is no egg,
spermatozoon, or sex.

It is a little mass of protoplasm containing a nucleus, and
corresponds, therefore, to one of the cells, most closely to the
egg-cell or spermatozoon of higher animals. If every living being is
descended from a single cell, the fertilized egg, it is not hard to
believe that all higher animals are descended from an ancestor
having the general structure or lack of structure of the amoeba.