THE ORIGIN OF INSECTS 39 



batch of ego\s. No doubt remains therefore that the Scotch 

 and southern insects are not distinct kinds, but varieties of 

 one kind. And instead of writing the name of the Scotch 

 insect as formerly , Potyommatus artaxerxes, we write Pol})- 

 ommatus astrarche yar. artaxerxes, the term variety being 

 applied where one form differs from another to a recog- 

 nisable extent, while, nevertheless, intermediate forms are 

 known to bridge over the gap between the two, and both 

 can be derived from the same parents. . . . Now it is 

 quite conceivable that the area where the typical astrarche 

 and the variety artc uveroces overlap might at some future 

 time be submerged beneath the sea, and so all the con- 

 necting links might become exterminated. Or the same 

 result might be brought about without any such serious 

 geographical change, since the dying out of several species 

 over wide areas has been noticed in recent years. The form 

 artaxerxes inhabits the western part of Ireland, in which 

 country the typical astrarche is not known to occur at all. 

 The Irish artaxerxes, then, is isolated from the English 

 astrarche by a sea-channel, and intermediate forms are 

 unknown. And if, as may happen in the future, the 

 Scottish artaxerxes should become similarly isolated and 

 the connecting links should die out, no hesitation 

 would be felt in considering the two insects as distinct 

 species." 



We see, therefore, that a species is, in its origin, a 

 variety which has become permanently estranged from its 

 near relations. This estrangement is usually so marked 

 that the individuals of one species will not — in the vast 

 majority of cases they cannot — interbreed with those of 

 another species. Yet it is often impossible to draw a 

 definite line between a species and a variety ; while a 

 species, although it may persist over long periods of time, 

 is in no sense immutable. In the course of ages it may 



