138 



DOMESTICATED ANIMALS AND PLANTS 



Fig. 22. A hand- 

 shaped corncob 

 showing a tendency 

 to branching of the 

 ear, not at all un- 

 common 



The stooling of grain is a case of branching at the base and 

 is a real doubhng, as are the four-, five-, or six-leaved clovers. 

 The whole matter of doubling is, of course, 

 the result of an extra cell division at the 

 proper point, — an abnormality that is some- 

 times hereditar}^ but oftener not, though a 

 strong tendency exists for any physiological 

 habit proceeding 

 from internal 

 causes to become 

 hereditary. 



Fusingof parts. 

 Quite the oppo- 

 site of doubling is 

 the fusing or joining of two parts 

 into one. Thus the two kidneys may 

 be joined at one end, making the 

 horseshoe kidney. A pair of horns 

 may be compounded into one. Two 

 fingers of the human hand or the 

 two toes of the pig ^ may be united 

 into one. 



When unit characters get mis- 

 placed. Perhaps the most remarkable 

 fact of development and differentia- 

 tion is seen when a normal struc- 

 ture develops in an abnormal place. 

 Thus occasionally a tooth will develop 

 in the roof of the mouth, as if the 

 germ' of it had in some way got 

 misplaced but was able to grow in 



its new place, like a tree that is transplanted. Sometimes the 

 eye of an insect will develop not as an eye but as an antenna. 



1 These are the so-called solid- or mule-hoofed hogs. This abnormality- 

 arises frequently and may be readily propagated, as it happens to be fairly 

 hereditary. See " Principles of Breeding," pp. 55, 66. 



Fig. 23. Compounding of paired 

 organs : the two horns of this 

 roebuck are united into a single 

 beam for a considerable distance, 

 but afterwards they separate. — 

 After Bateson 



