No. 607] 



RATS AND EVOLUTION 



407 



morphic species. Whereas polymorpliy in autogamous 

 plants really means the existence of a multitude of pure 

 lines, a great many closely related pure species of plants 

 which can easily be seen to originate through occasional 

 crossbreeding, polymorphic species in animals and wild- 

 flowering plants seems to be fundamentally different, in 

 that there is a continual crossbreeding going on without 

 the corresponding automatic i^urifieation which we see 

 everwhere. Such species as the ruff and some grouses 

 are always as variable as ever. The street-dog popula- 

 tion or the population of non-selected cats in any large 

 town will furnish examples of polymorphic animal spe- 

 cies. 



Now one of the chief factors in the diminishing of the 

 total potential variability of a group is certainly the fact 

 that a given number of animals in one population are cer- 

 tainly not the descendants of a number of parents of the 

 same magnitude, but of a very mucli smaller number of 

 parents. 



And it is easily conceived how the fact that every 

 female mates with several different males at each concep- 

 tion changes this disparity. This would partly account 

 for the continued existence of poljonorphy in the ruff and 

 in the cats and dogs whose breeding is unrestricted, and 

 in the sugar beet. 



As to the reality and the limitations, and differences of 

 species, the only way to reach a satisfactory conclusion 

 is breeding them. And the possibility of breeding rats 

 under scientific control is one of the reasons why so much 

 of the experimental probing into the species question is 

 connected with rats. 



The Javanese house-rat hns n uniformly dai-k IhAW, 

 dark feet and a long tail, and a fcrtaiti ri*I-v on thi^ ])ai-i- 

 etalia which no other Javaiic-f rat i>,,-^..^^f^, Out-o in a 

 while a rat is caught in a house or a - i t oan- 



house) with a short tail, or a soin.>\v _ , ,,r 



with marking^^ on tlio liind ffct. or wl: i;v 

 a study of these individuals only, it is imi-o-^i;,.- u> rhid 



