INTRODUCTION i 
that it contained a miniature adult, and that the process of 
development consisted essentially in enlargement and completion 
in detail of that which was already preformed. They solved the 
problem of development, therefore, by denying its existence: 
In the begininng the Creator had not only made all species of 
animals and plants in essentially their present forms, but had 
at the same time created the germs of all the generations that 
were ever to come into existence. The ovum of any species, 
therefore, contained encapsuled the germ of the next generation; 
this, likewise encapsuled, the germ of the generation next follow- 
ing, and so on to the predetermined end of the species. This 
was known as the doctrine of evolution or preformation. In 
opposition to this conception, those of the same period who be- 
lieved in epigenesis maintained the apparent simplicity of the 
germ to be real, and development to be actual. But, as there 
was no conception of the continuity of generations, the adherents 
of this point of view had to assume the spontaneous generation 
of the embryo. 
A great advance over the preformation theory of develop- 
ment was made in the modern theory of determinants. This 
conception, which forms the basis of Darwin’s theory of pan- 
genesis as well as of Weismann’s germ-plasm theory of develop- 
ment, is, essentially, that all the diverse components of the 
organism are represented in the germ by distinct entities (pangens 
of Darwin, determinants of Weismann) which are germs of the 
parts that they represent, and which are so distributed in the pro- 
cess of development that they produce all the parts of the embryo 
in their proper sequence and relations. This is not the place 
to enter into the numerous and diverse variations of the deter- 
minant hypothesis. It was an advance over the preformation 
theory of development in so far as it was reconcilable with the 
cell and protoplasm theories of organization, but it has a real 
relationship to the preformation theory inasmuch as it denies 
the simplicity of the germ and avoids any real explanation of 
the modus eperandi of development. 
Development is as truly a physiological process as secretion, 
and as such is to be studied by similar methods, mainly experi- 
mental. The limits of pure observation without experiment are 
soon reached in the analysis of such a complex subject as the 
physiology of development; experiment then becomes necessary 
