The Method of Evolution 51 



guinea-pig may be either rough or smooth (Figs. 21-24). 

 Rough is a unit-character dominant over smooth in crosses, 

 and among the second generation offspring from such a cross 

 occur three rough individuals to one smooth one. If the 

 rough parent is white (Fig. 23) and the smooth one dark 

 (Fig. 21), the parents differ in two unit-characters, and the 

 sequel shows that these are independent units. For 

 although the immediate offspring are all dark and rough 

 (Fig. 24), the next generation contains four sorts of indi- 

 viduals, representing all possible combinations of the two 

 alternative pairs of units: (i) smooth dark, like one grand- 

 parent (Fig. 21); (2) rough white, like the other (Fig. 23); 

 (3) rough dark, like the parents (Fig. 24) ; and (4) smooth 

 white, a new combination (Fig. 22). 



Again, length of the hair is independent of its color or 

 roughness (Figs. 25-27). A short-haired colored animal 

 mated with a long-haired white one produces only short- 

 haired colored offspring, which, bred inter se, produce hi 

 the next generation young of four sorts: (i) long white, 

 (2) short dark, (3) long dark, and (4) short white. 



Recombinations in such ways can be accounted for if 

 we suppose each different unit-character to have its basis 

 in a different material body within the cell, perhaps in a 

 different chromosome or part chromosome. Thus, suppose 

 hair length to have its basis in one cell structure, which we 

 may represent in a diagram (Fig. 29) by a circle, and sup- 

 pose hair color to have its basis in another cell structure 

 represented by a square, and suppose further that these 

 two structures are independent of each other in their fusions 

 and segregations; then if we cross a long-haired white (L W) 

 animal with a short-haired dark one (SD\ a combina- 

 tion (or zygote) will be formed showing only the dominant 



