94 THE GROWTH OF GROUPS 



There is no reason to believe that the change came 

 gradually. Suppose, for example, that there are a thousand 

 white hairs in the breast patch of such a marked animal, 

 there is no reason to believe that the number became 

 increased by small increments from one to a thousand in 

 a successive series of ancestors. The breast mark is 

 usually about an inch long and a quarter as broad in the 

 adult and occupies the middle line of the breast between 

 the fore-legs,' not always symmetrically. Rats marked 

 in this manner were found in various parts of India, for 

 example, Madras, Calcutta, Bombay, Poona, Nagpur. 



The mark may be smaller or larger than usual, it may 

 be a small spot or a long line two inches in length. If 

 we could obtain some hundreds of individuals marked 

 in this manner we should find, no doubt, that the mark 

 showed fluctuating variation as regards size ; but it is 

 evident that the character is appearing independently 

 in places remote from one another and when it appears it is 

 more likely to appear in what may be called the usual 

 amount, than in any other amount greater or less. 



When the occurrence of this breast mark was first 

 noticed, the fact did not seem to be of much interest. 

 The mark evidently appeared on various occasions, but 

 there was no evidence to show that it was transmissible 

 in full measure from parent to offspring ; nor did it seem 

 likely that such a character would appear in every 

 member of a large group so as to constitute a racial mark. 



Later on, however, evidence was obtained that rats 

 breed true in this respect, and it appears that the mark 

 was made use of as a specific character in the case of the 

 Irish rat Mus hibernicus. 



At Poona, four young rats were caught together in a 



