REPRODUCTION 91 



such, for instance, as two black guinea-pigs, 

 may produce offspring some of which may be 

 black and some white. In other words, mem- 

 bers of one elementary species may have in 

 their immediate offspring members of other 

 elementary species. Thus the idea of contin- 

 uity of breed, usually attached to the defi- 

 nition of a Linnaean species, is quite absent 

 from that of the elementary species. But the 

 main fact that underlies the whole De Vriesian 

 conception is the fixity of unit characters. 

 That there is a large body of truth in this, for 

 certain characters at least, cannot be denied, 

 but that the difference is comparable with 

 that between chemical elements, as originally 

 claimed by De Vries, is far from probable. 

 Evidence on this point comes from a number 

 of sources, but I shall cite only one instance. 

 The hooded rat is a domesticated form in 

 which the fur is dark over the head and down 

 the middle of the back, but is otherwise Hght- 

 colored. Thus the rat has the appearance of 

 a light animal wearins" a dark hood. When 

 this stock is crossed with the uniformly dark 

 wild rat, the hooded character is found to be- 

 have as a Mendelian recessive ; that is, in the 

 first generation of descendants all individuals 



