[Sidenote: Evolution and natural selection do not necessarily go together.] Many years before Mr. Spencer's day, it had been suggested, vaguely, that advancement seemed to be the great law of nature. Students of botany and zoology were especially struck by this fact, for they observed how animals and plants could be made to change and improve under favorable conditions, by the intervention of man's protection. In 1859, Mr. Charles Darwin published a theory to account for such variation, in which he assumed that there is a tendency on the part of all organisms to adapt themselves to their surroundings, and to change their characteristics, if necessary, in this attempt. He further showed that in the struggle for existence among animals and plants, the individual best fitted for its environment usually survives. These facts, Mr. Darwin thought, led to a process of natural selection, by which, through long ages, deep changes were caused in the structure of animals. In fact, Darwin held that the present-day plants and animals have descended from extinct and very different ancestors.[A] The experiences of daily life bear out the assertion that organic forms may be changed greatly--witness the breeding of stock and crops, practiced by all intelligent farmers--and all in all the theory seemed so simple that numerous biologists immediately adopted it, and began to generalize upon it. Having once accepted the principle that the present-day species have descended from very unlike ancestors, it was easy to assume that all organic nature had descended from one common stock. It was claimed that man, in a distant past, was a monkey; still earlier, perhaps, a reptile; still earlier a fish, and so on. From that earliest form, man had become what he is by a system of natural selection. In spite of the absence of proofs, such ideas became current among the scientists of the day. In this view was included, of course, the law of evolution or growth, and thus, too, the law became associated with the notion that man has descended from the lower animals. In fact, however, the law of evolution is just as true, whether or not Darwin's theory of natural selection be adopted. [Footnote A: Origin of Species, p. 6.] In justice to Darwin, it should be said that he in nowise claimed that natural selection was alone sufficient to cause the numerous changes in organic form and life; but, on the contrary, held that it is only one means of modification.[A] [Footnote A: Origin of Species, p. 6; also Darwin and After Darwin Romanes, Vol. II. pp. 2-6.]