PERPETUATION OF LIVING BEINGS. 105 



teristics 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 char- 

 acteristics, 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 dif- 

 ferent as species in many physiological characters. I 

 have already pointed out to you very briefly the differ- 

 ent habits of the breeds of Pigeons, all of which depend 

 upon their physiological peculiarities, — as the peculiar 

 habit of tumbling, in the Tumbler, — the peculiari- 

 ties of flight, in the "homing" birds, — the strange 

 habit of spreading out the tail, and walking in a pecu- 

 liar fashion, in the Fantail, — and, lastly, the habit of 

 blowing out the gullet, so characteristic of the Pouter. 

 These are all due to physiological modification, and in 

 all these respects these birds differ as much from each 

 other as any two ordinary species do. 



So with Dogs in their habits and instincts. It is 

 a physiological peculiarity which leads the Greyhound 

 to chase its prey by sight, — that enables the Beagle to 

 track it by the scent, — that impels the Terrier to its 

 rat-hunting propensity, — and that leads the Retriever 

 to its habit of retrieving. These habits and instincts 

 are all the results of physiological differences and pecu- 

 liarities, which have been developed from a common 

 stock, at least there is every reason to believe so. But 

 it is a most singular circumstance, that while you may 

 run through almost the whole series of physiological 

 processes, without finding a check to your argument, 

 you come at last to a point where you do find a check, 

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