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 



Fig. 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 ; ^ indeed, there is almost 

 no organ or part of the body that may not in rare instances 

 be doubled 2 (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 (see Fig. 19). 



1 " Principles of Breeding," pp. 44, 64, (i-]. 



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. 



