56 THE GROWTH OF GROUPS 



the same building. This being so, one would expect 

 to observe some interesting facts when the attention 

 was concentrated upon the members of one species in 

 particular, for similarly we should expect to find that 

 those lesser peculiarities, commonly spoken of as variation 

 within the species, would also be perceptible in particular 

 groups which were living and breeding together. This 

 was undoubtedly the case, as may be illustrated by the 

 following example. Melanotic specimens of Mus con- 

 color are not common. It was possible to examine a 

 thousand or more animals of this species without finding 

 one black one among them ; however, on the few occasions 

 when black rats of this species were to be seen at the 

 collecting stations, they were brought, two or three at a 

 time, by one and the same collector. Observations of 

 this kind were frequently made. It was evidently a 

 common occurrence for rats of a particular house to 

 possess certain special features, such as uncommon size, 

 unusual length of ear, of foot, tail, or whisker, a reddish 

 or yellowish tint in the fur, a tint which may be spoken 

 of as darker than usual, or to be pure black. This 

 state of things might be expected in any community 

 where inbreeding was rife. Observations of this kind, 

 though interesting at the moment, are not of value for 

 our demonstration, which is to show how a new group 

 of animals each possessing some obvious and special 

 characters of their own can arise among an old group of 

 animals which did not possess those characters. In 

 order to demonstrate this, it is necessary to know the 

 history of certain groups in detail. An opportunity for 

 making precise observations of this kind arose in the 

 following manner. Although most of the rats were 



