LIMITS or M.:VERSION: TIIP. SEA AS A BARRIER.
Ella kind of animal must have laid it point of origin, whence it spread as it increased. Theo retically, the expansion of a species would pro ceed equally in all directions, but actually this expansion has encountered barriers and restric tions that have con tined and shaped habitats within certain areas, in some instances extremely wide, in others surprisingly narrow. What eon stitutes these barriers and restrictions? That depends in each ease on the physical surumnd ings as related to the needs and abilities of the animal in question. it is plain that a species of fish that originated in or somehow became restricted to an inland water Lake Baikal, for example—could never spread beyond its shores save by accidental transplantation; while an able sea-going fish may wander indefinitely, no tar as mere room is concerned. Among land animals space for expansion, then, is of prime importance. Here the firmest bounds are set by
the sea. Any considerable breadth of water, and especially of salt water, is uncrossable except by rare and extraordinary accident: and even then a pregnant female or a pair must he landed on the further shore in order to start a colony, which must. furthermore, find favorable sur roundings in order to survive in the new locality. This accounts for the fact that mammals and terrestrial reptiles and amphibians are absent from oceanic islands. Continuity of land, then, is necessary to the spread of a terrestrial species; and when species are found in regions now widely separated. as Europe and North America, it can usually be shown that such regions were formerly connected by lands now submerged. Conversely, the characteristics of the. present fauna of such comparable regions assist the geologist to determine when the connection was finally broken.