LAW OF ASSOCIATION IN THE ANIMAL KINGDOM. 
41 
animal kingdom, including and governing those laics of growth, 
of organic repetition , and of economy, which have long since 
impressed the minds of physiologists, explaining the homologies, 
hitherto so mysterious, which are observed between the different 
parts of the body and the different kinds of limbs of the same 
animal, and bringing into a single group all the forms of 
sexual generation, which are its most powerful means of 
creation. Supported upon the law of the physiological division 
of labour, the importance of which was first demonstrated by 
M. Milne-Edwards, and upon that of polymorphism , which, 
without it have no precise meaning, and can only have a very 
restricted bearing, and in its turn the consequence of the law of 
division of protoplasmic masses, it has been the great producer of 
organisms, and establishes, in an unexpected manner, a fresh 
bond between sociology and those branches of biology which 
relate to the constitution and functions of organisms. 
We now come to the ultimate elements of living bodies, the 
materials which have served for the construction of the simplest 
of them, and we may ask ourselves what may have been their 
origin. Here we are in the presence of unity; there is no 
longer any question of association. Most living cells consist 
of four parts ; an enveloping membrane and semi-fluid contents, 
in the midst of which we observe a peculiar globule, the 
nucleus, itself containing a nucleolus. Of these four parts, 
onty one is truly indispensable, namely, the semi-fluid contents, 
which are either perfectly limpid, or finely granular, the 
protoplasm. It is in this curious substance that resides life, 
which has no need of any other apparatus to manifest its 
essential characters. Some particularly remarkable creatures, 
the Monera, are exclusively formed of it. These are simple, 
perfectly homogeneous clots of a limpid jelly, like white of 
egg. This jelly, nevertheless, performs movements, captures 
animals, digests them, incorporates them with its substance, 
enlarges, and divides, when it has attained a certain size, into 
two or more small masses, which recommence the life of their 
parent, and like it divide when they have reached a certain 
size. 
This faculty of division is an important property of proto- 
plasm, for it governs all organic evolution. A protoplasmic 
mass cannot exceed a determinate size. When it attains this 
size it divides, and as its mass is perfectly homogeneous, as it 
is constantly acted upon by currents which mingle the different 
parts of its substance, the fragments produced by this division 
possess all the acquired or inherited properties of the proto- 
plasmic clot of which they formed parts. This is the whole 
explanation of the important phenomenon of heredity, by virtue 
of which every organism transmits to its progeny (even in the 
