In your idea of species as specific amount or kind of concentrated force, you fall back upon the broadest and most fundamental views, and develop it, it seems to me, with great ability and cogency.
Taking the cue of species, if I may so say, from the inorganic, you develop the subject to great advantage for your view, and all you say must have great weight, in “reasoning from the general.”
But in reasoning from inorganic species to organic species, and in making it tell where you want it and for what you want it to tell, you must be sure that you are using the word “species” in the same sense in the two, that the one is really an equivalent of the other. That is what I am not yet convinced of. And so to me the argument comes only with the force of an analogy, whereas I suppose you want it to come as demonstration. Very likely you could convince me that there is no fallacy in reasoning from the one to the other to the extent you do. But all my experience makes me cautious and slow about building too much upon analogies; and until I see further and clearer, I must continue to think that there is an essential difference between kinds of animals or plants and kinds of matter. How far we may safely reason from the one to the other is the question. If we may do so even as far as you do, might not Agassiz (at least plausibly) say, that as the species Iron was created in a vast number of individuals over the whole earth, so the presumption is that any given species of plants or animals was originated in as many individuals as there are now, and over as wide an area, the human species under as great diversities as it now has (barring historical intermixture)?--so reducing the question between you to insignificance, because then the question whether men are of one or of several species would no longer be a question of fact, or of much consequence.
You can answer him from another starting-point, no doubt; but he may still insist that it is a legitimate carrying out of your own principle....
The tendency of my mind is opposed to this sort of view; but you may be sure that before long there must be one more resurrection of the development theory in a new form, obviating many of the arguments against it, and presenting a more respectable and more formidable appearance than it ever has before....