No. 607] 



RATS AND EVOLUTION 



387 



extraordinarily great number of keen-eyed persons, public 

 health officials, anxious owners of coffee-plantations, 

 managers of sugar factories, native officials in rice-grow- 

 ing centers, are continually observing the animals, and are 

 more than willing to collect extraordinary large numbers 

 on request. It is not uncommon for any one studying rats 

 to see several hundred animals brought together for him 

 to look over, and one of us has had the occasion to observe 

 a batch of ten thousand rats in one day within the grounds 

 of a sugar factory where between nine and twelve thou- 

 sand rats were killed daily for several years. 



The study of rats has set several authors to speculate 

 as to the nature and the origin of species. Very prominent 

 amongst these is Lloyd (The growth of groups in the 

 animal kingdom). Our condusions differ materially 

 from those of Lloyd, however. The reason for this dif- 

 ference, we venture to think, lies chiefly in the fact that 

 whereas Lloyd studied dead rats, and speculated upon the 

 origin of his animals, more especially of aberrant types, 

 we have been breeding rats for some six years, and have 

 witnessed the origin of aberrant types. The examples in 

 this paper will be found to be nearly all taken from rats. 



When it is found in field work, that two species-names, 

 each given to a skin in a museum drawer, in reality corre- 

 spond to two real groups in nature, of which they are 

 representative, we may be dealing with one of two dif- 

 ferent possibilities. It may be that the variability within 

 the first group is not so great that individuals belonging 

 to it fall within the limits of variability of the second 

 group, or it may happen that two different skins in a 

 museum belong to one highly variable group of animals, 

 in which it is difficult to establish dividing lines. If, for 

 instance, two skins with different names in a museimi 

 differ considerably in size, it may happen that even the 

 largest animals of the group to which the smallest skin 

 belongs are still very much smaller than the smallest 

 adult individuals of the group which corresponds to the 

 bigger skin. It may happen that two skins are consider- 



