HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 7 



Like ourselves, long-haired guinea-pigs bear hairs of both sorts, whereas 

 short-haired guinea-pigs, like our probable simian ancestors, bear only hairs 

 of determinate growth. Similar alternative conditions of hair-growth occur 

 in many different kinds of mammals, as for example in rabbits, goats, cats 

 and horses, the indefinite growth in the latter case occurring in the hairs 

 of the mane and tail only (see Davenport, 104) . The long-haired (or indefi- 

 nite growth) condition is doubtless ccenogenetic in all cases, and has probably 

 arisen independently in each case as a discontinuous variation or mutation. 

 By selection the long-haired condition is easily made a racial character, 

 for long-haired animals produce only long-haired offspring. But without 

 selection, what would be the fate of this mutation? This our breeding 

 experiments may perhaps indicate. 



Let us consider first the extreme conditions of follicle activity which 

 we have encountered in the guinea-pigs studied. In long-haired guinea-pigs 

 the hairs all over the body attain a greater length than in short-haired ones, 

 but only hairs of the back and sides grow indefinitely. Accordingly, in com- 

 parative studies of the hair-length, it has been found convenient to select a 

 few of the longest hairs to be found on the back or rump of the animal and 

 to use these as a standard of comparison. Following this method the hair 

 of guinea-pigs of different races has been measured at frequent intervals 

 from birth to an age at which the hair had attained full growth. 



A litter of four guinea-pigs of a pure short-haired race yielded measure- 

 ments which combined are expressed graphically in Fig. 2, A. Starting at 

 birth with a length of about 18 mm., the hairs grew very rapidly during the 

 first week, at the end of which they measured about 25 mm., an average 

 increase of a millimeter a day ; in the second and third weeks they grew less 

 rapidly, measuring about 29 mm. at the end of two weeks, and 33 mm. at the 

 end of three weeks. At this time the growth of the hairs which the animals 

 bore at birth was practically complete ; the hair- follicles now ceased to form 

 medullary substance, and consequently the hair narrowed to a base of solid 

 cortex (Fig. i). From this time on the measurements show from week 

 to week only slight deviations from a length of 33 to 35 mm. 



Several series of measurements of the hair of long-haired guinea-pigs, 

 when combined and expressed graphically, are shown in D, Fig. 2. The 

 hair-length at birth is about the same as in animals of class A, or even a little 

 shorter, but the growth rate does not show the series of rapid changes seen 

 in class A. Growth progresses very steadily at an average rate of about 0.83 

 mm. a day during the first one hundred days and at about 0.75 mm. a day 

 during the second one hundred days. But a glance at Fig. 2, D, shows 

 that the slowing up of the growth rate is a very gradual process. 



After the hairs have attained a length of about 170 mm. they begin to fall 

 out, but without previously narrowing to a base, as in class A. Rarely does a 



