No. 607] RATS AND EVOLUTION 4C9 



cure standing of varieties. If all the dwarf mice in a 

 given haystack have white tail tips, because of the fact 

 that the first two mice which happened to find the stack 

 when it was newly made had white tail tips, we can not 

 say that we are dealing with a new species. We have a 

 white-tailed variety of the local species. But if we take 

 a dozen of these mice into our house and succeed in breed- 

 ing them in cages, we may say that now we have founded 

 a domestic species. This species will continue to exist 

 as long as men will keep dwarf mice in captivity (witness 

 the so-called Irish rats) and long after the stack is broken 

 up, and the few remaining white-tailed mice have been 

 taken up into the normal species. The difference between 

 species and varieties is not determined by the magnitude 

 of the departure from a given type, and it is not a genet- 

 ical difference. It is a difference in expected permanency. 

 Varieties can become species by migrating into new sur- 

 roundings, or by a change in surroundings. 



It seems more than probable that a great many species 

 in museums are nothing but aberrant types which fall out- 

 side the normal variability of an existing species, and 

 have originated by crossing, one or more generations re- 

 moved. 



As we have already said, the only way to find out 

 whether individuals intermediate between existing species 

 have to be looked upon as hybrids, or descendants from 

 hybrids, or as variants of one of the species,- is by pro- 

 ducing the hybrids, and comparing them to the collected 

 material. 



It is rather difficult to get rats of the Eattus group to 

 breed in captivity. As we did not succeed in the begin- 

 ning, we rented a small vaulted room in the ruins of a 

 castle in France, fitted it out with numerous old boxes and 

 baskets, faggots and straw, and turned two females loose 

 in it with one male. There we gave them enough food to 

 last them for a week a< to (li>iHrl> tlu'iii a< little as pos- 

 sible. 



Later on, in Bussuiii, HoUand, we siK^crcded in breed- 



