x7. 46.] TO JAMES D. DANA. 431 
bound to try all the steps of the reasoning, and all the 
facts it rests on, impartially, and even to suggest all 
the adverse criticism I can think of. When I read 
the pamphlet I jotted down on the margin some notes 
of what struck me at the time. I will glance at them 
again, and see if, on reflection, they appear likely to 
be of the least use to you, and if so will send them, 
taking it for granted that you rather like to be criti- 
cised, as I am sure I do, when the object is the surer 
establishment of truth. 
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 experi- 
ence 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 
