THE PERPETUATION OF LIVING BEINGS, 109 



stand 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 characteris- 

 tics 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 cha- 

 racteristics, 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 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. 



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- 



