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 doubling, 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 hereditary but oftener not, though a 

 strong tendency exists for any physiological 

 habit proceeding 

 from internal 

 causes to become 

 hereditary. 



Fusing of 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 1 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 1 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 



