The Making of Species 
little room for doubt that it would be seized upon 
and preserved by natural selection, whenever it 
occurred. 
As E. H. Aitken very truly says, “so early 
and useful an invention should, one would think, 
have been spread widely in after time; but 
there appears to be some difficulty in developing 
muscles at the thin end of a long tail, for the 
animals that have turned it into a grasping organ 
are few and are widely scattered. Examples 
are the chameleon among lizards, our own little 
harvest mouse, and, pre-eminent among all, the 
American monkeys” (Strand Magazine, Nov. 
1908). 
Even as there are many variations which seem 
never to occur in nature, so are there others 
which occur so frequently that they may be 
looked for in any species. Albinistic forms 
appear now and again in almost every species 
of mammal or bird; while melanistic sports, 
although not so common, are not by any means 
rare. 
Every complete manual on poultry gives for 
each breed a note of the faults which constantly 
appear, and which the fancier has to watch care- 
fully for and guard against. The fact that these 
“faults” occur so frequently in each breed shows 
how strong is the tendency to vary in certain 
definite directions. It is true that some of these 
faults are in the nature of reversions, as, for 
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