596 VICTOR E. SHELFORD 
always necessary also. It is only under unusually favorable 
conditions that the relative importance of the various periods of 
the life-history of an animal can be ascertained, without experi- 
mentation. On the other hand, experimentation must be cor- 
related with field observation. Simple experimentation on the 
behavior of animals in the laboratory does not illuminate this 
matter to any appreciable extent. 
3. The relation of physiological characters to geographic range 
Our studies of animal distribution usually consist of a list of 
names of species with a statement of the distribution of each, 
followed by such interpretation as suits our particular purposes. 
Attempts actually to study the environment in any detail, or the 
reactions of animals to the conditions of environment are rare 
indeed. Furthermore, the groups most studied (higher verte- 
brates) are probably least dependent upon their environmental 
complexes; they are often decidedly migratory and because of 
their size least adapted to experimental study. 
Some quite extensive attempts to correlate geographic range 
with meteorological conditions have been made, but always with 
only implied reference to the physiological character of the organ- 
isms themselves, and usually with the use of species as an index 
of conditions. A few factors have been emphasized, and these 
usually in the sense of barriers. Merriam (’90, naming also 
Allen and Verrill but not citing their papers) emphasizes temper- 
ature; Walker (’03) atmospheric moisture. Heilprin (’87, p. 39), 
like most paleontologists, emphasizes food. There appears to 
be no adequate basis for the idea that the same single factor 
governs the distribution of most animals. Such a conclusion 
probably results from leaving the organism out of consideration. 
Since the environment is a complex of many factors, every 
animal lives surrounded by and responds to a complex of factors, 
at least in its normal life activities within its normal complex 
(Jennings, ’06, p. 180). Can a single factor control distribution ? 
A large amount of physiological study of organisms has been 
conducted with particular reference to the analysis of the organ- 
ism itself, but with little reference to natural environments. 
