294 _  OREATION BY LAW, 
can consist, and in the Fan-tail we have probably 
reached that limit. Many birds have the cesophagus 
or the skin of the neck more or less dilatable, 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 manner the differences in the size and form 
of the beak in the various breeds of the domestic 
Pigeon, is greater than that between 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 be- 
tween species and species in a state of nature, since 
the differences which we do produce are often com- 
parable with those which exist between distinct genera 
or distinct families. The facts adduced by the writer 
of the article referred to, of the definite limits to va- 
riability in certain directions in domesticated animals, 
are, therefore, no objection whatever to the view, that : 
all the modifications which exist in nature have been 
produced by the accumulation, by natural selection, of 
small and useful variations, since those very modifi- 
cations have equally definite and very similar limits. © 
Objection to the Argument from Classification. 
To another of this writer’s objections—that by Pro- 
fessor Thomson’s calculations the sun can only have 
