Thomas Henry Huxley 
Conditions of Existence as Affecting the Perpetuation of Living Beings [EPUB ebook] 

pub One.info present you this wonderfully illustrated edition. IN the last Lecture I endeavoured to prove to you that, while, as a general rule, organic beings tend to reproduce their kind, there is in them, also, a constantly recurring tendency to vary- to vary to a greater or to a less extent. Such a variety, I pointed out to you, might arise from causes which we do not understand; we therefore called it spontaneous; and it might come into existence as a definite and marked thing, without any gradations between itself and the form which preceded it. I further pointed out, that such a variety having once arisen, might be perpetuated to some extent, and indeed to a very marked extent, without any direct interference, or without any exercise of that process which we called selection. And then I stated further, that by such selection, when exercised artificially- if you took care to breed only from those forms which presented the same peculiarities of any variety which had arisen in this manner- the variation might be perpetuated, as far as we can see, indefinitely