310 GERM-CELL CYCLE IN ANIMALS 



FIG. 84. Diagrams illustrating the union of two stocks with paired 

 factors A, B, C, D, and a, b, c, d, to form pairs Aa, Bb, Cc, Dd. 

 Their possible recombinations are shown in the sixteen smaller 

 circles. (After Wilson.) 



must accept these facts of heredity as a basis. If 

 evolution is a fact, new species must have arisen from 

 time to time. This may have occurred by the drop- 

 ping out of old factors or the addition of new factors. 

 There seems to be sufficient evidence that factors 

 are sometimes left out, but there are very few cases 

 of the addition of new factors. Our ideas of a pro- 

 gressive evolution demand the addition of new factors, 

 but whether this is brought about by changes within 

 the germ-plasm or is the result of external influences 

 is not known. 



