44 THE GROWTH OF GROUPS 



or part come to be in that state ? We are dealing here 

 with the portion or group denned as Mus rattus, and 

 we must inquire as to the state of the group at the 

 moment. 



First, let us consider a few numerical facts. The 

 number of house rats in India may be estimated roughly 

 at a thousand millions. It could not be less than this. 

 The estimation is based on the assumption that on the 

 average there are about three rats for every person in 

 any house or village. The house rats of India, then, are 

 a large number at the moment, and if we wish to know 

 how many have been born in the last ten years we must 

 multiply that number by fifty or more. But what is our 

 warrant for knowing the state of this mass at the moment 

 or during the last ten years? It is this. Before the 

 year 1905, perhaps as many as a thousand Indian house 

 rats had been examined in the scientific manner. Since 

 that year, several thousands have been examined or 

 looked at ; but it is evident that the amount of our 

 observation upon the mass is trivial in comparison with 

 the size of the mass. The amount observed is a minute 

 fraction of that not observed, but yet we must form a 

 mental picture of the mass as it is, before we inquire 

 how it came to be. 



A specific name for the mass and for every member of 

 it is merely a restful convention. In applying the term 

 rattus to the Indian house rat as a whole, we make the 

 tacit assumption that all of them are alike, or rather, we 

 agree to ignore any points of difference that may be 

 detected among them. For certain purposes this con- 

 vention is sufficient and useful, but it is not a true descrip- 

 tion of what can be seen. 



