136 DOMESTICATED ANIMALS AND PLANTS 
Doubling of parts. One of the most common of abnormalities 
is the increase of numbers of parts, especially by doubling. An 
extra finger or toe is by no means rare, and double thumbs or 
even double hands are not unknown (see Fig. 20). 
The horse, having developed from a five-toed ancestor, has 
frequently an extra toe or two, as does the cow, though less 
commonly. Certain strains of sheep have four horns instead 
LANA 
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Fic. 20. Symmetry within the variable part. Here it would seem that an attempt 
has been made to repeat the hand, or rather that an attempt at repetition of the 
thumb has resulted in a doubling of the hand. — After Bateson 
of two, and occasionally the deer shows a cluster of horns instead 
of the normal growth. 
Insects frequently double a leg or a wing, and turtles and 
snakes occasionally double the head ;1 indeed, there is almost 
no organ or part of the body that may not in rare instances 
be doubled ? (see Fig. 21). 
Doubling among plants is exceedingly common, being noth- 
ing more or less than branching. Double clover heads are 
found everywhere, double timothy heads rarely, and double 
wheat still more rarely. Double ears of corn, or, more properly, 
“fingered” ears, are frequently found, as are little ears on the 
end of the tassel (sec Fig. ry). 
1" Principles of Breeding,” pp. 44. 64, 67. 
2 The so-called double-headed people of the shows have been in every case 
really twins united by fleshy growth, an abnormality that occasionally happens. 
