THE GROWTH OF GROUPS 25 



parent species. It is for attributes such as melanism 

 that I use the term character, attributes that we may 

 recognize as present or absent. 



THE NUMERICAL CONSTITUTION OF GROUPS 



Let us now return to our definition of the term group. 

 A group is a number of individuals each possessing some 

 particular character or characters which are the dis- 

 tinguishing marks of the group. 



A group is a number. How large a number ? Can 

 two form a specific group ? can ten ? The answer would 

 usually be given without hesitation in the negative. 

 But what answer shall we give to the question can a 

 thousand individuals form a specific group ? The answer, 

 though hesitating perhaps, would be in the affirmative ; 

 for, if not, the number specified in the question might be 

 raised until the question be answered in the affirmative. 



If then there are a thousand organisms in proximity 

 on a certain part of the earth's surface, each possessing 

 a particular combination of characters which are not 

 found in other animals, they will be collectively a specific 

 group. Let us suppose that such a group is found in 

 nature, its numerical strength being recorded at a certain 

 moment, and again after an interval of several years. 

 It may be found at the second census that the numerical 

 strength of the group has increased a thousand-fold. 

 This increase would not excite wonder, for it is well 

 known that an expansion of the kind has occurred in the 

 history of certain groups, and we may be sure that such 

 might occur in many cases, provided that the interval 

 of time between the two enumerations was sufficient, 



