V.] SPECIFIC STABILITY. 131 



treme 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 6 of 

 these developments : " Variation seems to have reached its 

 limits in these birds. But so it has in nature. The fan- 

 tail 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 fan- 

 tail we have probably reached that limit. Many birds have 

 the oesophagus, or the skin of the neck, more or less dilata- 

 ble, but in no known bird is it so dilatable as in the pouter 

 pigeon. Here again the possible limit, compatible with a 

 healthy existence, has probably been reached. In like man- 

 ner, the difference in the size and form of the beak in the va- 

 rious breeds of the domestic pigeon, is greater than that be- 

 tween the extreme forms of beak in the various genera and 

 sub-families of the whole pigeon tribe. From these facts, and 

 many others of the same nature, we may fairly infer that, 

 if rigid selection were applied to any organ, we could, in a 

 comparatively short time, produce a much greater amount 

 of change than that which occurs between species and spe- 

 cies in a state of nature, since the differences which we do 

 produce are often comparable with those which exist be- 

 tween distinct genera or distinct families." 



But, in a domestic bird like the fantail, where Natural 

 Selection does not come into play, the tail-feathers could 

 hardly be limited by " utility for flight," yet two more tail- 

 feathers could certainly exist in a fancy breed, if " utility for 

 flight " were the only obstacle. It seems probable that the 

 6 " Natural Selection," p. 293. 



