DIVISIONS OF ZOOLOGY. 5 
seem almost certain that they have a crystalline structure. 
There are, however, no grounds for supposing that the mole- 
cules of protoplasm and chlorophyll have any definite form. 
Multiplication of Cells—A living corpuscle of proto- 
plasm is constantly undergoing irregular contractions, and 
these contractions are occasionally so great as to divide the 
corpuscle into two parts. These parts may break away from 
each other, and thus become two independent living beings, 
or they may be held together by secreted matter and thus 
form a tissue. The causes of the contractions which produce 
division of the cells are not yet known, but no doubt 
temperature and the supply of nourishment are two very 
important factors. In all cases it is the living matter that 
divides, and in this way life is transmitted from one being 
to another. All the knowledge we have on the subject goes 
to show the impossibility of life being derived in any other 
way, and thus the rule “omne vivum e vivo” has come to 
be one of the most firmly established principles of Biology. 
When the two divided masses are nearly equal, the process 
of division is called fission ; when one is much smaller than 
the other, it is called gemmation. 
DIVISIONS OF ZOOLOGY 
Animals may be studied from three different points of 
view, and therefore the science of Zoology can be conve- 
niently broken up into three divisions, called respectively, 
Morphology, Physiology, and Distribution. 
Morphology is the study of the form and structure of 
animals; the shape and arrangement of the parts, both 
external and internal. It is entirely observational in its 
method. As, however, the shape and arrangement of the 
parts undergo many and great changes in the early life of 
