That many and serious objections may be advanced against the theory of descent with modification through variation and natural selection, I do not deny.

I have endeavoured to give to them their full force.

Nothing at first can appear more difficult to believe than that the more complex organs and instincts have been perfected, not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor.