THE GROWTH OF GROUPS 23 



of Poona do not appear to be characters. The dis- 

 tinction is of course assumptive. The probable difference 

 between the two kinds of attributes may be illustrated 

 in the following manner. If all the rats of Poona were 

 to be exterminated and the town restocked with rats from 

 Belgaum, it is probable that after a few generations had 

 passed the descendants of the new-comers would be 

 rather small and dark like the old stock which had been 

 exterminated, the change being due to some local con- 

 dition. Peculiarities of this kind have been called 

 " place variation," they certainly appear to be of a 

 different order from characters such as those of jerdoni. 

 Surely an extra palatal ridge must be something different 

 from peculiarities of this kind. At present, however, 

 we have no knowledge of the grounds of the difference. 



It may be mentioned in passing that some taxono- 

 mists regard peculiarities, such as are exhibited by the 

 rats of Poona, as specific. Having satisfied themselves 

 that the animals of a certain area have a perceptible 

 peculiarity, however slight, so long as it is visible and 

 constant among the group, they apply a specific name 

 to them. From a practical point of view species are 

 those groups which have been described as such. If, 

 then, using the term in this sense, we inquire into the 

 origin of species it might be possible to explain the 

 origin of some without explaining that of all. It might 

 be possible to explain how the rats of Poona became small 

 and dark but impossible to explain how jerdoni obtained 

 an extra palatal ridge and a flat tympanic bulla. 



Most of our defined species, however, are groups 

 of animals, each possessing a particular combination of 

 characters which appear to be as distinct as those of 



