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XI PHENOMENA OF ORGANIC NATURE 421 
briefly the different habits of the breeds of 
Pigeons, all of which depend upon their physio- 
logical 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 modifi- 
cations, 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 been developed from a common stock, at 
least there is every reason to believe so. But it 
isa 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, and that is in the reproduc- 
tive processes. For there is a most singular cir- 
cumstance in respect to natural species—at least 
about some of them—and it would be sufficient 
