Mere chance ... alone would never account for so habitual and large an amount of difference as that between varieties of the same species.

Species of the same genus would occasionally exhibit reversions to lost ancestral characters.

On the ordinary view of each species having been independently created, we gain no scientific explanation.

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is evidently not arbitrary like the grouping of stars in constellations.

Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult, at least I have found it so, than constantly to bear this conclusion in mind.

If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case.