( xxxiii ) 



produced two ver}' different kinds of caterpillars, from wbich he obtains in the 

 course of time a series of imagines again verj' mucli alike, but presenting now 

 to his searching eye and suspicious mind some slight differences. Continued 

 experiment with the two insects proves to him that he has to do, not with a 

 dimorphic larva, but with two entirely indejiendent beings, which fly together 

 and feed as larvae on the same plant, and which are scarcely distinguishable 

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

 that there is a gap between these two kinds of insects which is utterly different 

 from the gap between the varieties which stand in the relation of parents and 

 offspring ; that there is an effective barrier which lies in the nature of the 

 specimens themselves, separating the two sets of individuals completely, though 

 there is no extraneous barrier between them. And by studying further the life 

 around himself, the student will perceive that the animated world is composed 

 of a multitude of such sets of specimens, of such entities, all separated by that 

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

 for the classifier. AVhat the barrier is the student cannot know with certainty. 

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

 are only accessory to it. 



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

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

 and some strangers may meet his 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 which were in the minority in the first place are here in a majority, 

 the mean of the variability having changed. Another entity, which was knov?n 

 to him as being monomorphic, is dimorphic in the 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 the student had hitherto seen, the range of variation 

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

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

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

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

 difference. 



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

 entities ? The two extreme cases are these : — 



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

 identical, with the 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 

 dili'ereuce may be found only iu 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. The difference may be structural, or chromatic, or both ; it may be 



c 



