INFLUENCE OF EXTENT OF RANGE 123 



dental transportation. The Norway rat, house mouse, and domestic 

 dog among mammals; the English sparrow among birds; the flea, the 

 meal worm, various ants (e.g., Camponotus rubripes) , and the house- 

 fly among insects; the slug Limax variegatus and the edible snail 

 Helix aspersa among mollusks; and Eisenia foetida and Helodrilus 

 caliginosus among earthworms, 7 may be cited as examples. 



The geologic age of a group has an important bearing on the extent 

 of its range. Highways for dispersal available for older forms may 

 have disappeared before the rise of younger groups. The families of 

 invertebrates, probably on this account, have wider ranges than those 

 of vertebrates. The wide distribution of so many genera of scorpions, 

 pedipalps, and centipedes is doubtless connected with their antiquity 

 and their now diminished variability. Among fresh-water mollusks, 

 the earliest known genera are likewise the most widely distributed, as 

 is illustrated by Planorbis, Physa, Limnaea, Ancylus, Unio, and 

 Pisidium, all of which are present in the Jurassic and some as early 

 as the Carboniferous. Anodonta, which does not appear until the early 

 Tertiary, has a much more restricted range. Greater geologic age seems 

 to be as important a factor in wide distribution as vagility. The 

 moths and butterflies are unquestionably more vagile than the beetles; 

 some of them are pronounced wanderers, and a few have a world-wide 

 range. This is a rare phenomenon among the species of beetles, but 

 it is a striking fact that nearly all the families of beetles have a 

 world-wide distribution, while this is not true of the majority of the 

 families of the Lepidoptera. The Coleoptera are an older group, 

 abundantly represented in the earliest stages of the Mesozoic, whereas 

 the Lepidoptera are unknown until Mid-Jurassic. Willis 8 attempts to 

 establish an invariable connection between extent of range and geo- 

 logic age of species, but is forced to admit that this relation may be 

 greatly veiled by the presence of physical and ecological barriers, by 

 the action of man, and by other factors, even among the slow-spreading 

 plants. A restricted range may be characteristic of groups at their 

 decline as well as at their inception, as is illustrated by the New 

 Zealand Sphenodon, and by the king crabs now confined to the east 

 coast of North America and to the Moluccas. 



The extent of the range of a given group of animals is also de- 

 pendent upon their "ecological valence." Adaptability of any sort 

 favors the establishment of a species in new districts and hence favors 

 the extension of its range. This relation is especially important for 

 terrestrial animals on account of the wide range of habitat conditions 

 on land. The widely transported inhabitants of temporary ponds are 

 eurythermal and frequently also euryhaline, like the salt-crustacean 



