If, in a state of nature, you find any two groups of living beings, which are separated one from the other by some constantly-recurring characteristic, I don't care how slight and trivial, so long as it is defined and constant, and does not depend 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. As regards the great majority of physiological characteristics, there is no doubt that they are capable of being developed, increased, and modified by selection. There is no doubt that breeds may be made as different as species in many physiological characters. I have already pointed out to you very briefly the different habits of the breeds of Pigeons, all of which depend upon their physiological peculiarities--as the peculiar habit of tumbling, in the Tumbler--the peculiarities of flight, in the "homing" birds--the strange habit of spreading out the tail, and walking in a peculiar fashion, in the Fantail--and, lastly, the habit of blowing out the gullet, so characteristic of the Pouter. These are all due to physiological modifications, and in all these respects these birds differ as much from each other as any two ordinary species do.