V.] SPECIFIC STABILITY. 131 



any animal (of a given size) moving over the earth's sur- 

 face ; also it is said that the differences in degree of change 

 shown by different domestic animals depend in a great 

 measure upon the abundance or scarcity of individuals 

 subjected to man's selection, together with the varying 

 direction and amount of his attention in different cases ; 

 finally, it is urged that the changes found in nature are 

 within the limits to which the variation of domestic ani- 

 mals extends, it being the case that when changes of a 

 certain amount have occurred to a species under nature, it 

 becomes another species, or sometimes two or more other 

 species by divergent variations, each of these species being- 

 able again to vary and diverge in any useful direction. 



But the fact of the increasing difficulty found in pro- 

 ducing, by ever such careful selection, any further extreme 

 in some change already carried very far (such as the tail 

 of the " fantailed pigeon " or the crop of the " pouter "), is 

 certainly, so far as it goes, on the side of the existence of 

 definite limits to variability. It is asserted in reply, that 

 physiological conditions of health and life may bar any 

 such further development. Thus, Mr. "Wallace says 1 of 

 these developments : " Variation seems to have reached its 

 limits in these birds. But so it has in nature. The fantail has 

 not only more tail-feathers than any of the three hundred 

 and forty existing species of pigeons, but more than any 

 of the eight thousand known species of birds. There is, 

 of course, some limit to the number of feathers, of which a 

 tail useful for flight can consist, and in the fantail we have 

 probably reached that limit. Many birds have the oeso- 

 phagus or the skin of the neck more or less dilatable, but 

 in no known bird is it so dilatable as in the pouter pigeon. 



i " Natural Selection," p. 293. 

 K 2 



