CHAPTER XV 
EXPERIMENTAL STUDY OF GROWTH 
INTRODUCTORY 
Normal Growth 
ALTHOUGH growth is recognized as one of the fundamental 
properties of living things, comparatively little zodlogical work 
has been done in this field. Botanists have paid more attention 
to the phenomena of growth and with marked success. In al- 
most every field of biological investigation the process of growth 
is directly or indirectly involved in the changes that take place; 
yet the connection between these changes and growth is often 
obscure, for as yet we know almost nothing in regard to what 
takes place in the protoplasm during growth, and very little in 
regard to the causes that incite growth or inhibit it. 
In this and in the following chapters I shall attempt to give 
some of the most suggestive results that have been obtained 
regarding the growth of animals, although the purely physio- 
logical side of the question, where most, in fact, has been 
accomplished, will occupy a secondary place. Attention will be 
directed more particularly to the gross phenomena of normal 
growth and to the external factors that influence the growth of 
animals. 
The process of growth may be said to begin with the egg and 
to end with the adult. While in some animals the adult condi- 
tion coincides in a very general way with the condition of sexual 
maturity, in other animals growth may continue throughout the 
length of life of the animal, becoming smaller in amount as the 
size increases beyond what we may speak of as the adult condi- 
tion. Ina general way we may class these two kinds of growth 
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