mother as any other of the associations concerned, and thereafter, whenever that particular sound strikes upon the child’s ear, the association of mother is aroused. Precisely the same sort of thing is true of written language. Arbitrarily selected symbols act through the sense of sight to arouse nervous disturbances, which are built into particular associations, and here again it makes no particular difference what the symbol is; one will do as well as another, provided a number of people have agreed to use that symbol to stand for the same thing. One very useful feature of language is that we can make a single word stand for a complex group of sensory impressions or even associations. To illustrate, the word physiology, whether spoken or written, is in itself simply a sound or a visible symbol, but it stands for a highly complex group of associations in the human mind. In this respect language is a kind of shorthand. It is hard to overestimate the importance of language to the human race. This is because of the enormous extension it gives to the ability to profit by experience. The lower animals are able to profit by experience, but the only experience that they can get is that which comes to them individually or to another animal which they may observe. Human beings, on the other hand, may profit by the experience of their contemporaries through learning of them by word of mouth, or may profit by the experience of present or past generations as recorded in writing. It is not too much to say that the progress of civilization depends on our ability to profit by the experience of past generations as made known to us in writing. We are further advanced than our remote ancestors, not because we are actually superior to them in mental power, but because we have a much larger background of experience than they had, and this background depends altogether on our ability to use language.