Characters which are acquired during the life of the individual owing to differences in the use of certain organs or of exposure to light, heat, drought, wind, moisture, etc., are comparatively very slight, and are liable to be so combined with _innate_ characters and with the effects of natural or artificial selection, that it is exceedingly difficult to ascertain, without such careful and long-continued experiments as have not yet been made, whether they are in any degree transmissible from parent to offspring, and therefore cumulative. Almost every individual case of supposed inheritance of such characters, when carefully examined, has been found to be explicable in other ways; but there is a very large amount of _general_ evidence, demonstrating that even if a certain small amount of such inheritance exists, it can certainly not be a factor of any importance in the process of organic evolution, all the factors of which must be universally present because the process itself is universal. I will therefore here limit myself to a short enumeration of a few of the very numerous cases in which the continued use of an organ does not strengthen or improve it, but often the reverse; and of others in which it cannot be asserted that the action of the environment can have had any part whatever in the continuous change or specialisation of the part or organ. The number, size, form, position, and composition of the teeth of all the mammalia are extremely varied, and throughout the whole class afford the best characters to distinguish family and generic groups; they are therefore of great value in determining the affinities of extinct forms, because the jaws and teeth, especially the latter, are most frequently preserved. But as the permanent teeth are always fully formed while buried in the jawbones and covered by the gums, it is quite certain that the special adaptation of the teeth of each species to seize, crush, tear, or grind up its particular food cannot possibly have been produced by the act of feeding, the effect of which is almost always to grind away the teeth and render them less serviceable. Such adaptation could not possibly have been produced by _use_ alone, or any other direct action of the environment. Yet, as the adaptation is clear, and often very remarkable, some eminent palæontologists have declared it to be proved that the changes in them were _produced_ by the changes in the environment, and that they constitute very strong evidence of the "inheritance of acquired characters"—a statement unsupported by any direct evidence. The same objection applies to most of the special organs of sense. The internal organ of hearing is a highly complex series of bones and membranes, protected by the outer ear; but it cannot be even imagined to have been gradually developed by the action of the air waves the vibrations of which it conveys to the brain.