Mr. Darwin, following Weismann, long ago pointed out that there are two main factors in all Evolution--the nature of the organism and the nature of the conditions. We have chosen our illustration from the highest or human species in order to define the meaning of these factors in the clearest way; but it must be remembered that the development of man under these directive influences is essentially the same as that of any other organism in the hands of Nature. We are dealing therefore with universal Law. It will still further serve to complete the conception of the general principle if we now substitute for the casual phrases by which the factors have been described the more accurate terminology of Science. Thus what Biography describes as parental influences, Biology would speak of as Heredity; and all that is involved in the second factor--the action of external circumstances and surroundings--the naturalist would include under the single term Environment. These two, Heredity and Environment, are the master-influences of the organic world. These have made all of us what we are. These forces are still ceaselessly playing upon all our lives. And he who truly understands these influences; he who has decided how much to allow to each; he who can regulate new forces as they arise, or adjust them to the old, so directing them as at one moment to make them coöperate, at another to counteract one another, understands the rationale of personal development. To seize continuously the opportunity of more and more perfect adjustment to better and higher conditions, to balance some inward evil with some purer influence acting from without, in a word to make our Environment at the same time that it is making us--these are the secrets of a well-ordered and successful life.