go THE OX AND ITS KINDRED 
Bell,^ if a " cow produce twin calves, one of each sex, 
the male is perfect, and the female barren ; which 
last is termed a free martin. This is so generally 
true, that there are not, I believe, more than two or 
three instances of its fertility." Still more rarely 
three calves are produced at a birth. 
Normally the central pair of lower milk-incisor 
or front teeth of the calf are shed and replaced by 
their permanent successors in the tenth month ; the 
adjacent, second, pair is replaced in the sixteenth 
month ; and by the third year the outermost, or third, 
pair, together with the adjacent and nearly similar 
pair of canines, has likewise been shed and replaced. 
Here again, however, selection and specialisation 
have produced abnormal acceleration in certain 
breeds, so that there is a difference of no less than 
six months in the date of the appearance of the 
various pairs of lower incisors.^ 
^ British Quadrupeds, 1st ed. p. 417, 1837. 
- Darwin, op, cit. p. 91. 
