INHERITANCE 



315 



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

 alternative inheritance characters behave as units, and, 

 more than that, as wholly independent units, so that to fore- 

 cast the outcome of matings is merely a matter of mathe- 

 matics. While this is in a measure true, it is, fortunately or 

 unfortunately, not the whole truth. In alternative inheri- 

 tance 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 as the 

 separation of the two glass plates. The union of maternal 

 and paternal substance in the germ-cells of the cross-bred 

 animal is evidently 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 certain to be in- 

 cluded in the other, so that w r hile the two strata retain their 

 identity, each is slightly modified by 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. 



"Cross-breeding, accordingly, is a two-edged sword which 

 must be handled carefully. It can be used by the breeder to 

 combine in one race characters found separately in different 

 races, but care must be exercised if it is desired to keep 

 those characters unmodified. If modification of characters 

 is desired at the same time as new combinations, then cross- 

 breeding becomes doubly advantageous, for it is a means of 

 inducing variability in characters, as, for example, in the 



