( xxxiii ) 



produced two very different kinds of caterjjillars, from wbicb lie obtains in tbe 

 course of time a series of imagines again very niucli alike, bnt presenting now 

 to liis searcbing eye and suspicious mind some sligbt differences. Continued 

 experiment witli tlie two insects proves to liim tliat lie bas to do, not witb a 

 dimorpbic larva, but witb two entirely independent beings, wbicb fly togetber 

 and feed as larvae on tbe same plant, and wbicb are scarcely distinguisbable 

 as adults, and are nevertbeless perfectly kept apart in nature. He realises 

 tbat tbere is a gap between tbese two kinds of insects wbicb is utterly different 

 from tbe gap between tbe varieties wliicb stand in the relation of parents and 

 offspring ; tliat tbere is an effective barrier wbicb lies in tbe nature of tbe 

 specimens tliemsclvcs, separating tbe two sets of individuals completely, tbough 

 tbere is no extraneous barrier between tbem. And by studying furtber tbe life 

 around bimself, tbe student will perceive tbat tbe animated world is composed 

 of a multitude of sucb sets of specimens, of such entities, all separated by tbat 

 same kiud of barrier. The knowledge of the existence of this barrier is essential 

 for the classifier. What the liarrier is the student cannot know witb certainty. 

 The corporeal differences observed in the individuals are not the barrier, but 

 are only accessory to it. 



Passing now into a neighbouring country, tbe scientist will find practically 

 the same composition of tbe fauna, tbough some old friends may be missing 

 and some strangers may meet bis eye. A good many of the entities will 

 indeed be identical with what he knew before, but others appear in an altered 

 garb. In one the range of variation has remained the same ; but the indi- 

 viduals wbicb were in the minority in the first place are here in a majority, 

 tbe mean of the variability having changed. Another entity, wbicb was known 

 to liim as being monomorphic, is dimorpbic in tlie new locality. A third, 

 which was seasonally variable there, does not exhibit seasonal variability 

 here. Among the specimens of a fourth entity there appear individuals 

 different from what tbe student had hitherto seen, tbe range of variation 

 having become shifted or widened. In others, again, he finds the proportion 

 of sucb different individuals to be larger and larger, until the student 

 comes to entities of which all specimens exhibit some distinction from the 

 individuals of tiie former country. They are the same entities, but with a 

 difference. 



Now, in which relation to each other stand sucb geographically separated 

 entities ? Tbe two extreme cases are tbese : — 



(1) The geographically separated entities are, each taken as a whole, 

 identical, witb tbe same range of variation and the same mean of variability. 



(2) The geographically separated entities are different from one another in 

 all individuals. 



Between these two extreme cases there are all intergradations. The 

 difference may be found only in a very small proportion of the specimens, 

 or only in one sex, or in the greater number of individuals, or in nearly all, 

 or in all. Tbe difference may be structural, or chromatic, or both ; it may be 



