This is a mere guess, but it gives the order of the magnitude." Now, if so, "how long would it take to obtain an elephant from a protozoon, or even from a tadpole-like fish?
Ought it not to take much more than a million times as long ?"[139] Mr.Darwin[140] would compare with the natural origin of a species "unconscious selection, that is, the preservation of the most useful or beautiful animals, with no intention of modifying the breed." He adds: "But by this process of unconscious selection, various breeds have been sensibly changed in the course of two or three centuries." "Sensibly changed!" but not formed into "new species." Mr.Darwin, of course, could not mean that species _generally_ change so rapidly, which would be strangely at variance with the abundant evidence we have of the stability of animal forms as represented on Egyptian monuments and as shown by recent deposits.

Indeed, he goes on to say,--"Species, however, probably change much more slowly, and within the same country only a few change at the same time.

This slowness follows from all the inhabitants of the same country being already so well adapted to each other, that places in the polity of nature do not occur until after long intervals, when changes of some kind in the physical conditions, or through immigration, have occurred, and individual differences and variations of the right nature, by which some of the inhabitants might be better fitted to their new places under altered circumstances, might not at once occur." This is true, and not only will these changes occur at distant intervals, but it must be borne in mind that in tracing back an animal to a remote ancestry, we pass through modifications of such rapidly increasing number and importance that a geometrical progression can alone indicate the increase of periods {139} which such profound alterations would require for their evolution through "Natural Selection" only.
Thus let us take for an example the proboscis monkey of Borneo (_Semnopithecus nasalis_).

According to Mr.Darwin's own opinion, this form might have been "sensibly changed" in the course of two or three centuries.
According to this, to evolve it as a true and perfect species one thousand years would be a very moderate period.

Let ten thousand years be taken to represent approximately the period of substantially constant conditions during which no considerable change would be brought about.