304 GENERAL BIOLOGY 



aggregate to be one species and the divergent types 

 to be " geographical races," or " varieties." The 

 criterion by which we decide whether or not a given 

 aggregate of individuals is a " true species," or only 

 a variety, is that of lack of intergradation of one 

 or more (not necessarily all) characters. But it will 

 be seen that the increase of our acquaintance with 

 a certain group of related species may necessitate 

 a constant revision and rearrangement of them. 

 Let us consider for a moment an imaginary species 

 of bird, extending, let us say, from the Atlantic coast 

 to the Rocky Mountains, representatives of which 

 show a wide range of measurements in the bill. 

 Suppose that the eastern representatives have a 

 bill averaging two centimeters in length, whereas 

 the western ones have a bill averaging four centi- 

 meters, but with every gradation between the two 

 extremes. Assuming that this is the only significant 

 difference between them, we should consider the whole 

 aggregate to be one species, with two well-marked 

 geographical varieties. But if some one were able 

 to willfully wipe out of existence the individuals 

 from the middle districts (as the passenger pigeon 

 has been wiped out within the memory of the present 

 generation), then a later naturalist, who was ignorant 

 of that fact, would be justified in considering the 

 eastern and western forms distinct species. On the 

 other hand, an earlier student who might have re- 

 tained specimens of the exterminated intermediate 

 types, would certainly continue to consider them all 

 one species. In a way, either would be right, for 



