12 HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 



is simply a poorly developed D, C and D being grades, arbitrarily fixed, of 

 the continuous growth type of hair. Or, if we think of C as a synthesis of 

 B with D, then this C partakes more largely of the character of D the 

 more often the parental B has been crossed with D in its production. From 

 the fact that the C parents give off gametes partaking in different degrees 

 of the D character, it seems probable that the synthesis of B and D to form C 

 has been as yet imperfect, as of two ingredients incompletely mixed together, 

 so that different samples contain different proportions of the D character. 

 This view is supported by the result obtained by mating C with C, but as 

 the parents used in this experiment were of different origin from those men- 

 tioned in the foregoing pages, it may be well first to describe the source 

 from which they came. 



4. ORIGIN OF RACE C', AND RESULTS OF CROSSES IN WHICH IT WAS 



EMPLOYED. 



A family of short-haired guinea-pigs (class B), when inbred, produced 

 a few individuals with hair about twice as long as that of their parents. 

 The long hairs were not very numerous. It seems probable, in the light of 

 subsequent studies, that only a few of them were continuous in growth, 

 the others being determinate, but attaining a greater length than do ordi- 

 nary hairs before growth ceased. When two of these long-haired animals 

 were mated together, all the offspring were long-haired, though some of 

 them had more numerous long hairs than others, or hairs of a greater maxi- 

 mum length. By selecting the best long-haired individuals for two genera- 

 tions a race of imperfectly long-haired individuals was produced, of about 

 the same degree of excellence as the group C already described, which was 

 produced by cross-breeding between B and D. To distinguish it from the 

 latter, we may call this race C'. 



The C race, bred by itself, produced no B individuals, though the 

 maximal hair lengths obtained varied considerably. This result shows 

 clearly that no B gametes were produced by the C' race, though it had just 

 arisen from the B race, in which very likely it had previously been present 

 as a recessive character. 



A mating between C' and D gave a mixture of C and of D individuals, 

 the two classes being about equally numerous (seven C to five D in one 

 set of experiments). Certain individuals, however, were just on the line 

 between the two classes, so that no great importance can be attached to 

 the proportions observed. But the result does show, what breeding C or C' 

 individuals inter se had shown, a considerable degree of variability among 

 the C and C' gametes, this variability being a sufficient basis for selection 

 for increased hair-length. While, accordingly, the variation made its appear- 

 ance as a discontinuous one, it showed itself amenable to selection. 



