Species 133 



while the caterpillar of astrarche feeds on rock-rose 

 and stork's-bill, that of the blue insect feeds mostly 

 on rest-harrow. No hesitation is felt therefore in 

 referring the latter to a distinct species, and it is 

 known to naturalists as Polyommatus icarus. The 

 second of the two names by which every animal is 

 called is the specific name ; astrarche and icarus dis- 

 tinguish respectively the two species which we have 

 considered. 



" Variety " and "Species." — We have seen that 

 the Scotch form {artaxer-xes) of P. astrarche was 

 formerly believed to be a distinct kind of butterfly 

 from the southern English type, and was only ad- 

 mitted to be a variety of the latter when intermediate 

 links connecting the two were found in northern 

 England. Now it is quite conceivable that the area 

 where the ranges of the typical astrarche and the 

 variety artaxerxes 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. It is impossible to draw 

 any hard and fast line between "species" and 

 "variety"; the use of these terms must depend on 



