440 BIOLOGY AND ITS MAKERS 
the biologist is to investigate the architecture of living beings; 
then there arise questions as to the processes that occur within 
the organism, and the study of processes involves the employ- 
ment of experiments. In the pursuit of physiology exper- 
iments have been in use since the time of Harvey, but even 
in that science, where they are indispensable, experiments 
did not become comparative until the nineteenth century. 
It now appears that various forms of experiment give also 
a better insight into the structure of organisms, and the prac- 
tice of applying experiments to structural studies has given 
rise to the new department of experimental morphology. 
For the purpose of indicating some of the directions in 
which biology has been furthered by the experimental method 
of investigation, we designate the fields of heredity and evo- 
lution, changes in the environment of organisms, studies on 
fertilization and on animal behavior. 
The recognition that both heredity and the process of 
evolution can be subjected to experimental tests was a revela- 
tion. Darwin and the early evolutionists thought the evolu- 
tionary changes too slow to be appreciated, but now we 
know that many of the changes can be investigated by 
experiment. Numerous experiments on heredity in poultry 
(Davenport), in rats, in rabbits, and in guinea-pigs (Castle) 
have been carried out—experiments that test the laws of 
ancestral inheritance and throw great light upon the ques- 
tions introduced by the investigations of Mendel and De 
Vries. The investigations of De Vries on the evolution of 
plant-life occupy a notable position among the experimental 
studies. 
A large number of experiments on the effects produced 
by changes in the external conditions of life have been made. 
To this class of investigations belong studies on the regulation 
of form and function in organisms (Loeb, Child), the effects 
produced by altering mechanical conditions of growth, by 
