In this way we acquire an approximate conception of the possible mechanical _modus operandi_ of actual events--namely, of the manner in which the useful variations required by the conditions of life _can_ always, that is, very frequently, make their appearance. This possibility is the sole condition of our being able to understand how different parts of the body, absolutely undefined in extent, can appear as variational units and vary in the same or in different directions, according to the special needs of the case, or as the conditions of life prescribe. Thus, for example, in the case of the butterfly's wings it rests entirely with utility to decide the size and the shape of the spots that shall vary simultaneously in the same direction. At one time the whole under surface of the wing appears as the variational unit and has the same color; at another the inside half, which is dark, is contrasted with the outside half which is bright; or the same contrast will exist between the anterior and posterior halves; or, finally, narrow stripes or line-shaped streaks will behave as variational units and form contrasts with manifold kinds of spots or with the broader intervals between them, with the result that the picture of a leaf or of another protected species is produced. I must refrain from entering into the details of such cases and shall illustrate my views regarding the color-transformations of butterflies' wings by the simplest {49} conceivable example--viz. that of the uniform change of color on the entire under surface of the wing.