RECENT DISCOVERIES IN HEREDITY. 205 



sive characters, any pair of individuals manifesting that combination 

 will breed true. But if the desired combination contains one or more 

 dominant characters, then each animal selected must be tested for the 

 presence of undesirable recessives before one can be sure that the new- 

 race will breed true. In practise it is found best by the breeder 

 not to work with too many characters at a time, but to eliminate 

 the undesirable recessives one by one. Otherwise the search for the 

 one individual in a large number which will breed true may prove a 

 long and tedious process. If we deal with one character at a time, the 

 chances are that one in four of the second generation of animals reared 

 will meet our ideal ; if we deal with two characters at a time, the chances 

 are one in sixteen ; while if we deal with three characters at a time the 

 chances are only one in sixty-four; and so on, with the chances of suc- 

 cess diminishing in a geometrical series. 



From what has thus far been said it would appear that in alter- 

 native inheritance characters behave as units, and, more than that, as 

 wholly independent units, so that to forecast the outcome of matings is 

 merely a matter of mathematics. While this is in a measure true, it is, 

 fortunately or unfortunately, not the ivhole truth. In alternative in- 

 heritance characters do behave as units independent of one another, but 

 the union of dominant character with recessive in a cross-bred animal is 

 not so simple a process as putting together two pieces of glass, nor is 

 their segregation at the formation of gametes so complete in many cases 

 ae: the separation of the two glass plates. The union of maternal and 

 paternal substance in the germ-cells of the cross-bred animal is evi- 

 dently a fairly intimate one, and the segregation which they undergo 

 when the sexual elements are formed is more like cutting apart two 

 kinds of differently colored wax fused in adjacent layers of a common 

 lump. Work carefully as we will, traces of one layer are almost cer- 

 tain to be included in the other, so that while the two strata retain 

 their identity, each is slightly modified b} r their previous union in a 

 common lump. 



Thus, when we cross short-haired with long-haired guinea-pigs, we 

 get among the second-generation offspring a certain number of long- 

 haired animals with hair less long than that of the long-haired grand- 

 parent, or with long hair on part of the body only (Fig. 13). Further, 

 certain of the short-haired animals have hair a little longer and a little 

 softer than that of the short-haired grandparent. Again, rough-coated 

 guinea-pigs produced by cross-breds often have coats less fully rough 

 than that of their rough ancestor, lacking certain of the typical rosettes 

 (Fig. 14). Finally, when an albino is crossed with a fully pigmented 

 animal, the result may be not a wholly pigmented animal, but one 

 spotted with white. While such a cross-bred animal forms a full quota 

 (one-half) of albino gametes, the pigment-bearing gametes formed by 

 it frequently bear this spotted or modified pigmented condition. 



