520 MERKEL HENRY JACOBS 
nature, are more difficult to deal with than morphological ones, 
and partly from the fact that single physiological characters at 
least, are notoriously unreliable guides in the questions of classifi- 
cation and phylogeny that up until the present day have occupied 
so large a share of the attention of working biologists. 
The latter objection, however, no longer holds today, at least 
to the same extent that it formerly did. Modern zoology is 
not so much interested in finding out what are the probable rela- 
tionships of a given animal as in learning what it is, and especially 
what it does. This is the physiological point of view, whichis 
uppermost in the minds of most biologists today. No data which 
throw light on what goes on in the living organism are any longer 
considered unimportant; indeed, they are coming to be recognized 
as a vital necessity. If our knowledge of comparative physiology 
were as complete as our knowledge of comparative morphology, 
for example, there is not a single one of the more modern develop- 
ments of biological science that would not have its possibilities 
enormously extended. It is therefore a matter of increasingly 
great importance to accumulate accurate data on the physio- 
logical characters of organisms, to determine which ones are 
fundamental, and which accidental, which are constant in a given 
species, or larger group, and which vary in different individuals 
of the same species, or perhaps in the same individual at different 
times; in short to obtain as full and comprehensive a knowledge 
as possible of the physiological characters of organisms. Perhaps 
the day may come when it will be possible to define any species 
in physiological and chemical terms in the same way in which it is 
now defined in morphological ones, and when no description of 
an organism will. be considered complete which does not include 
its chief physiological peculiarities along with its structural one. 
The biologists of that day will be able successfully to attack prob- 
lems that for the present must remain untouched on account of | 
lack of the proper kind of knowledge. 
It is needless to state that many observations of the sort sug- 
gested have already been made. Not to mention the more or 
less scattered ones made on many widely separated groups of 
organisms, we already have a considerable knowledge of the 
