200 THE SCIENTIFIC MONTHLY
tively be considered as a '^species consisting of a number of ^'indi- viduals undergoing reproduction by the process of mitosis. There are two very distinct environmental forces in the life of such an embryo. One we may call the '^ external environment, as exemplified by the surroundings of the embryo, the other we may call the internal en- vironment" which includes the relationships between cells (indi- viduals) within the embryo. During the gastrula stage the internal environment is relatively simple, but as the embryo grows we find that complexities appear one after another. As the number of cells (indi- viduals) increase, we find that the number of types of cells (species) increase as well (differentiation of tissues). This may be considered largely the result of differences in the internal environment in which certain cells or groups of cells find themselves. Nutrition, and un- doubtedly to a large extent internal secretions, play the leading parts among the influences of the internal environment.
The young mammal shortiy before sexual maturiiy has thus reached a point where a steady process of cell division (multiplication of indi- viduals) within many definite types of tissue (many species) is in progress. Now into this more or less balanced condition is introduced the secretions of the newly active sex glands, ovaries or testes, as the case may be. At once the internal environment is fundamentally changed. By the circulation the modifications introduced by these secretions are transmitted through the body, reaching all types of cells in all localities. It is as though in a given isolated geographic unit, populated by a fauna of many species, a certain food tree was intro- duced in great numbers in addition to the somewhat similar types of food trees formerly there. This new tree provides food which gives certain species of animals in certain localities more suitable nutrition than they have yet obtained. The result is rapid growth and repro- duction of that particular species, while the others near it may be un- affected, or may even suffer by the rapid multiplication of the favored species.
It is obvious that changes in the internal environment will be fre- quent. The cyclic changes of the reproductive system, including also the changes of pregnancy and of lactation, undoubtedly represent fun- damental upsets of the equilibrium of the internal environment. The same, of course, holds for retrogressive changes such as accompany the cessation of activity of the reproductive system and the progressive changes of approaching senility.
To any biologist, it will have long ago suggested itself to question the influence of the inherent physico-chemical nature of the cell ma- terial. Undoubtedly this is a matter of fundamental importance, for it is in the reaction of the cells to the influences and agencies of the internal environment that initiation, continuation and control of cell
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