on sexual peculiarities, then all naturalists agree in calling them two species; that is what is meant by the use of the word species--that is to say, it is, for the practical naturalist, a mere question of structural differences.*
[footnote]* I lay stress here on the 'practical' signification of "Species." Whether a physiological test between species exist or not, it is hardly ever applicable by the practical naturalist.
We have seen now--to repeat this point once more, and it is very essential that we should rightly understand it--we have seen that breeds, known to have been derived from a common stock by selection, may be as different in their structure from the original stock as species may be distinct from each other.
But is the like true of the physiological characteristics of animals? Do the physiological differences of varieties amount in degree to those observed between forms which naturalists call distinct species? This is a most important point for us to consider.