CREATION BY EVOLUTION
more than Rome was, but they are being made here and now, and many biologists are studying the steps, conditions, and causes of evolutionary changes in animals and plants. All such study of contemporary evolution is necessarily a study of successive generations of individuals, and all analytical or experimental study of the causes of evolution resolves itself into a study of the factors involved in the genesis of individuals; there is no other possible method of approaching the problem. The study of the factors involved in the genesis of individuals under various conditions of inheritance and environment reveals all that can certainly be known regarding the methods and causes of the evolution of races and species.
The causes of the development of an individual or of the evolution of a species are twofold, internal and external. The internal causes are represented by the organization of the germ cell, the external by surrounding conditions; the internal causes may be called heredity, the external causes environment.
An egg cell, like every other kind of cell, functions in response to stimuli. When a muscle cell is stimulated it contracts, when a gland cell is stimulated it secretes, when an egg cell is stimulated it develops. The stimulus comes, in the first instance at least, from the environment; an egg will start to develop only when stimulated by a spermatozoon or by certain salts or chemicals, or by changes in temperature, and it will continue to develop only so long as environmental stimuli of water, oxygen, food, temperature, etc., remain favorable to its development.
The character of development depends primarily upon the nature (that is, the hereditary organization) of the egg concerned, and secondarily upon the environmental stimuli. The former determines all the possibilities of development
[ 76 ]
