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 thoseobserved 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 habitof 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.