Shoulder-pad. is the thick, almost horny skin on the shoulder

of the wild Boar, which is said to serve as defence to tusks of other boars: I have not examined this myself.—²

Cross & intercross.— There is no difference: I ought not to have used both terms: an English friend has complained of this mistake of mine.—³

Structure anything visible to eye in appearance of parts

Constitution. rate of pulse—period of gestation—length of life—liability to certain diseases—degree of fertility &c &c &c.

Habit—whether diurnal or nocturnal—what kind of food an animal prefers— Habit may, also, be applied to instincts; but it is rather a loose word, often used in English. It differs little from instinct; but would never be applied to the more wonderful instincts. We might say that the owl had nocturnal habits; but instinct impelled it to build its nest.— The optic nerve being fitted to see in the dark we should call a constitutional difference—the great size of eyes a structural difference.—

I am astonished & much pleased that you think the Translation will be ready by May.—⁴ Most truly do I thank you for the great honour & benefit which you have conferred on me.—

Honoured Sir, Yours very sincerely | C. Darwin