COAT CHARACTERS IN GUINEA-PIGS AND RABBITS. 65 



is then gradually shed. On the other hand, the hair of long-coated ani- 

 mals apparently is not shed at this period, but keeps on growing. 

 At two months of age it is 5 to 7 cm. long, at three months it is 6 to 

 9 cm. long. In the period from three to four months of age is another 

 critical stage. Animals which begin to shed their longest hair at this 

 period apparently do not acquire a longer coat at any period of their 

 subsequent life, but continue to have one whose maximum length is about 

 8 cm. But animals which pass the age of about four months without 

 shedding their longest hairs increase the length of their coat rapidly, 

 so that they may have a coat of 10 to 1 2 cm. maximum length when four 

 to five months old, and one of 14 to 16 cm. at six to seven months. 



The acquisition of an abnormally long coat is due, accordingly, not 

 to unusually rapid growth of the hair, but to a peculiar mode of growth. 

 The hairs of a normal animal are thick and stiff in the middle, but taper 

 toward either end, somewhat like the quills of a porcupine. In the 

 middle of the hair the medulla is very thick, but it diminishes gradually 

 toward either end, where it is wanting altogether. A hair of this sort, 

 then, is the result of a definite growth cycle in the hair follicle, this 

 cycle covering apparently about two months' time, though my observa- 

 tions on this point are yet imperfect. When a hair is completed, after 

 attaining a length of about 4 cm., a new one apparently is formed below 

 it and crowds the old one out. But in the long-haired animal, the 

 termination of a two-months growth cycle is frequently, if not regularly, 

 omitted. The hair is more nearly of uniform thickness throughout its 

 length, and is not narrowed to a base at the end of a single period of 

 two months, but grows without interruption during two, three, or four 

 such periods. 



For this reason, as I have observed, hair measurements of long- 

 coated animals do not form a continuous series, but tend to group 

 themselves about mean maximal lengths as follows : 



(i) S cm., double the length of normal or short coat, not attained 

 under three months of age. 



(a) 12 cm., three times the length of short coat ; not attained under 

 five months of age. 



(3) 16 cm., four times the length of short coat; not attained under 

 seven months of age. 



Coats longer than this I have not had an opportunity to study, though 

 such are said to occur. 



That long-haired animals bred inter se produce only long-haired 

 young is shown by the following experiment : Six different long-haired 

 males (viz, 955, 1586, PI. 5, fig. 9, 1709, 2002, PI. i, fig. i, and 2060, 

 PI. 3, fig. 5) have been mated with 19 different long-haired females, 

 producing 49 young, all long-haired. There were produced also seve- 



