This substance, which is manifold in its forms and protean in its transformations, has, in its state of living matter, one physiological name which has become familiar, that of protoplasm.

We have spoken of beings so low in the scale that the individuals throughout their whole existence are not sufficiently specialized to be distinctively plant or animal: yet these are definite life in simpler shape.

Natural selection is not the wind which propels the vessel, but the rudder which, by friction, now on this side and now on that, shapes the course.

Agassiz, when I saw him last, had read but a part of Origin of Species. He says it is POOR-VERY POOR!!. The fact is, he is very much annoyed by it.

Indeed upon much that may have to say, I expect rather the charitable judgment than the full assent of those whose approbation I could most wish to win.

Your candor is worth everything to your cause. It is refreshing to find a person with a new theory who frankly confesses that he finds difficulties, insurmountable, at least for the present.

The former conviction that these two kingdoms were wholly different in structure, in function, and in kind of life, was not seriously disturbed by the difficulties which the naturalist encountered when he undertook to define them.