The geological evidence for the gradual changes in life on the earth's surface is so strong that nobody who has studied it can deny its reality or its profound meaning. Many theories have been proposed to account for the remarkable phenomenon of this evolution of life on earth.
Since we know that "life begets life," and that the offspring is very similar to its parents, the first obvious explanation is that there is a change in the offspring, that the latter acquires in some way or other new properties or faculties. St. Hilaire developed the theory that if the change was unfavorable the individual died and disappeared, whereas, if the change was in a favorable direction with regard to the environmental conditions, the individuals live and multiplied. There are some objections to this theory. One is that the monstrous forms, which sometimes appear in breeding and which St. Hilaire thought were of special importance in evolution, rarely if ever furnish individuals that have any advantage over their parents. In practically all cases they are unfit for life, at least in the wild state. Nevertheless, there are elements of truth in his theory as will later appear.
Bergson, in recent times, has suggested that life responds to environmental conditions in such a way that the offspring is often modified in a direction which we might well regard as favorable. It puts the cause of the change in a mystical [?] property of life, the "élan vital," which conception does not explain the actual process or cause of development.
Lamarck started from the fact that the use of an organ improves it while disuse makes it less efficient. If the improvements could be inherited, an organ, like a bird's wings, could perhaps gradually be developed. But such qualities are in general not inherited. Further, if we waved our arms for a million generations, we could not develop wings with feathers ; at the most we might develop strong arms. We may perhaps improve our vision by training, but even if such improvements were inherited, we could still not understand the develop met of an eye out of "nothing."
The theory has also been advanced that all the faculties of present-day life were inborn in the first life on earth. The favorable characters have been "unfolded," the unfavorable ones have been lost with the disappearance of unfit animals and plants.
Darwin, whose theories have been so widely discussed, regarded the chance variations in plants and animals as the primary elements in evolution. These
variations may be helpful or harmful for the individual in the struggle for existence, and in the former case the variations, some of which he supposed to be hereditary, were actually incorporated in the race. (Etc.)
Darwin was particularly looking for the cause of the variations, and we now turn to a search for this cause. Most variations are not inherited, but there are types of variations, which biologists call mutations, which are actually inherited.