256 Agassiz 1 Contributions to the 



our supposing that they may have proceeded originally from a 

 single being or a single pair." This being admitted, it must be 

 only on the most cogent grounds, to be established in every indivi- 

 dual case, that we can admit a difference of origin either in geo- 

 logical time or in space, for animals that on comparison appear to 

 be specifically identical. 



It may be objected that this is a merely hypothetical definition ; 

 but we contend that it is as practical as the opposite view, that it 

 is indeed essential to any trustworthy determination of species. 

 If we have given to us a number of individuals absolutely similar, 

 we do not doubt their specific unity, or, as we even sometimes 

 venture to call it, identity ; but if there are differences which we 

 suppose may be specific, we inquire whether these differences ex- 

 ceed those known to occur in individuals of common parentage. 

 If w;- are informed that these same diversities occur in individuals 

 of the same brood or litter, in individuals that have been trans- 

 ferred to different conditions of life, or in individuals of different 

 age or sex, we discard them as specific distinctions. If we cannot 

 obtain these facts as to the species in question, we compare large 

 numbers of specimens to ascertain the gradations that occur, or 

 we refer to the known facts in allied species, or in those which 

 may be supposed similar in tendency to variation. We always 

 suspect determinations which, on the one hand, require us to be- 

 lieve specific diversity in forms no farther apart than those known to 

 be connected by parentage ; or, on the other, unity where the dif- 

 ferences are greater than this. Other considerations, of course, 

 enter into such questions ; but the identity of the protoplast, or 

 mould, is one essential element in our complete mental conception 

 of the species. 



We could, on the other hand, state practical evils injurious to 

 the mere technical accuracy of Zoology, likely to arise from the 

 opposite view. One of these may suffice. It is their tendency to 

 take it for granted that forms must be new specifically, merely 

 because they are found in new places — a mischievous laxity 

 likely to prevail where so loose views as to species are held by a 

 great leading naturalist ; too wise himself to be so misled, but 

 unable to communicate his own largeness of mind to followers 

 who eagerly adopt a view tending to increase their chance of be- 

 coming species-founders, or at all events their freedom to commit 

 errors in this matter, without being liable to the charge of sepa- 

 rating individuals connected by actual descent from common an- 

 cestors. 



