82 CONDITIONS AFFECTING THE 



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 charac- 

 teristics, 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. 



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 peculiarities, which have 

 l>een developed from a common stock, at least there is 

 y reason to believe so. But it is a most singular 

 circumstance, that while you may run through almost the 

 whole scries of physiological processes, without finding 

 a check to your argument, you come at last to a point 

 where you do find a check, and that is in the reproductive 

 processes. For there is a most singular circumstance in 

 respect to natural species -at least about some of them 

 and it would be suflicient for the purposes of this argument, 

 if it were true of only one of them, but there is, in fact, a 

 great number of such cases and that is, that similar as 



