No. 607] BATS AND EVOLUTION 415 



bellied agouti, and one agouti with lemon-yellow belly. 

 As females 24 and 25 were litter sisters, with the same 

 parentage, we may be allowed, for the present discussion, 

 to add their young together. There were 37 young, of 

 which 14 were blacks and 17 agoutis (expected 15.5 and 

 15.5). Of the 37, five were yellow, one pearl gray, two 

 with white tail tip, one white-throated, four waltzers, and 

 one yellow-bellied, all of which are animals with totally 

 new characters. 



We can easily explain the origin of the new characters 

 as follows. If both parent species possess a gene, which 

 by its presence or absence makes the difference between 

 a normal and a waltzer, or in other words, if to be normal 

 a rat's germ must at least possess either Y or Z, the 

 hybrids, which are impure for Y as well as for Z, having 

 inherited Y from one and Z from the other parent, will 

 produce one germ-cell in every four, from which both Y 

 and Z are lacking. Therefore such hybrids will produce, 

 when mated among themselves, fifteen normal young and 

 one waltzer in every sixteen. If we expect the same rea- 

 soning to hold good for a number of new recessive char- 

 acters, which are displayed by neither of the parents, so 

 that animals lacking W and X will be yellows, others, 

 lacking JJ and V, will have white-tipped tails, we should 

 in our case expect to find among our thirty-seven young, 

 two to three (2.312) with the new character in every case. 

 In reality we found yellow five, pearly gray one, white 

 tail tip two, white throat one, waltzers four, yellow belly 

 one, that is 2.33 on the average. 



These numbers make it clear that we are not dealing 

 with a sort of period of mutation ; it was easy to see that 

 the new types were already given in the genotype of the 

 three species crossed. 



Female no. 24 later was mated back to her son no. 95. 

 From this mating we obtained among a number of normal 

 rats, one chocolate and two pearl-gray young. Later we 

 obtained a cinnamon agouti rat, that is to say an animal 

 that probably stands genot>i)ically in a relation to agouti, 



