Page:Popular Science Monthly Volume 27.djvu/812

790 surmise or demonstrate their causes. It is evident that, without the genealogical lines which paleontology presents, it is impossible that our hypotheses on this subject can rest on any solid basis. With these lines completed, we will be able, on the other hand, ultimately to reach a demonstration in most if not all the cases which present.

In the first place, there has long been before the world a theory to account for these changes, and that is the doctrine of use and disuse, propounded by Lamarck. He believed that the use of a part of an animal caused it to grow larger, and that in consequence of disuse a part would grow smaller or become extinct. Another theory is frequently spoken of, as though it accounted for the origin of changes, and that is the Natural Selection of Darwin. While naturalists are generally agreed as to the importance of this principle in modifying the results of the creative energy, but few of them regard it as explaining in any way the origin of the changes with which it deals. In the very nature of things, "selection" can not act until alternatives have been presented. And Mr. Darwin and others, who treat of natural selection as though it were a cause of new structures, always premise by admitting that "there is in organic beings a natural tendency to variation." It is in accounting for this variation that the Lamarckian hypothesis is useful, and probably expresses a great law of organic nature. In the same way most of those who write of the "influence of the environment" (which Lamarck, by-the-way, fully considered), as though it embraced the causes of evolution, forget that the energy which impresses an animal from this source could have no effect unless the animal possess some impressibility or capacity for response. And they also often forget that an animal capable of free movement is able to modify its environment very materially.

There is one element of weakness in the Lamarckian theory of use. This is that use implies the presence of something to use, or the existence of a usable part or organ. It is thus incompetent to explain the origin of such a part ab initio, although it may account for the details of its structure, as its segmentation, branching, etc. So I have added to use the energy of effort, and the Lamarckian theory, completed, can be characterized as the theory of the origin of species, by effort, use, and disuse. A prominent cause of change of structure may be here referred to; and that is, change in consequence of excess or diminution of growth-energy, due to the action of use and effort in disturbing its equilibrium. That is to say, that excessive growth in one place has caused diminished growth in another, and vice versa. This derivative hypothesis explains the origin of many structures which are not useful to the possessor, and of others which appear to be positively injurious. Such characters can not, of course, be accounted for by the direct action of effort and use, which are necessarily directed toward beneficial ends. There are also numerous characters, chiefly of an ornamental nature, as color, etc., which are the direct result of the