Now the word _development_, which like many other long words has become very common, is also, like many other words, not unfrequently used somewhat loosely. The root of it, the word _velop_, is unknown in any other form than the two words _envelope_ and _develope_.[61] In mathematics, the word _develope_ is used, as all words are, with the utmost precision. We speak of _developing a function_, that is, putting it into a new and unfolded form, which, however, shall be essentially equivalent to the original. So also we speak of _developable surfaces_, that is, surfaces such as cones and cylinders, which can be unfolded and laid flat upon a plane without tearing. It will be seen that in these applications of the word the essential thought is that of a change, by a process of unfolding, in the condition of something which you already possess; and this I take to be the true definition of development. From this, however, we easily pass to a cognate meaning of the term. Thus we speak of the development of an idea, that is, the unfolding and applying of the results of an original thought, a discovery or principle, which were truly contained in it from the first, but were not from the first perceived to be so contained. For example, we say that railways are only a development of the original idea of turning to account the expansive force of steam; or that Newton's "Principia" and Laplace's "Mecanique Celeste," and, in fact, the whole of modern physical astronomy, are developments of the idea, or fact, call it which you will, of the universal gravitation of matter; or that the British constitution of this century is a development of _Magna Charta_; and so forth. What we mean by this language is that the essential principles of the development were implicitly contained in the original idea, and that one has been derived from the other somewhat in the same way as that in which the bird comes from the egg and the plant from the seed.