=Significance of Development and Causality.= What are the consequences of the view just set forth? What is the significance of the facts that have been enumerated? It is of great consequence to our thinking when we come to recognize fully the idea of causality. We then fully accept the fact that man's body and mind are part of a causal and orderly world. Let us consider, for example, the movement of a muscle. Every such movement must be caused. The physiologist has discovered what this cause is. Ordinarily and normally, a muscle contracts only when stimulated by a nerve current. Tiny nerve fibrils penetrate every muscle, ending in the muscle fibers. The nerve-impulse passing into the fibers of the muscles causes them to contract. The nerve stimulus itself has a cause; it ordinarily arises directly or indirectly from the stimulation of a sense organ. And the sense organs are stimulated by outside influences, as was explained previously. Not only are our movements caused, but our sensations, our ideas, and our feelings follow upon or are dependent upon some definite bodily state or condition. The moment that we recognize this we see that our sensations, ideas, and feelings are subject to control. It is only because our minds are in a world of causality, and subject to its laws, that education is possible. We can bring causes to bear upon a child and change the child. It is possible to build up ideas, ideals, and habits. And ideas, ideals, and habits constitute the man. Training is possible only because a child is a being that can be influenced. What any child will be when grown depends upon what kind of child it was at the beginning and upon the influences that affect it during its early life while it is growing into maturity. We need have no doubt about the outcome of any particular child if we know, with some degree of completeness, the two sets of factors that determine his life--his inheritance and the forces that affect this inheritance. We can predict the future of a child to the extent that we know and understand the forces that will be effective in his life. The notion of causality puts new meaning into our view of the _training_ of a child. The doctrine of development puts new meaning into our notion of the _nature_ of a child. We can understand man only when we view him genetically, that is, in the light of his origin. We can understand a child only in the light of what his ancestors have been.