More Letters of Charles Darwin Volume 2

I am sorry to say that I cannot give you the least aid, as I have never
attended to any mechanical subjects. I should doubt whether it would be
possible to train birds to fly in a certain direction in a body, though
I am aware that they have been taught some tricks. Their mental powers
are probably much below those of mammals. It is said, and I suppose
truly, that an eagle will carry a lamb. This shows that a bird may have
great power for a short distance. I cannot remember your essay with
sufficient distinctness to make any remarks on it. When a man is old and
works hard, one subject drives another out of his head.