The geographical distribution of animals, with a study of the relations ...

chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 47families) would have a limited range ; but, as is to some extentthe case in tropical America* east of the Andes, every suchgroup would be represented over the whole area, by countlessminute modifications of form adapted to local conditions.2. One great physical barrier would, however, even thenexist ;the hot equatorial zone would divide the faunas and florasof the colder regions of the northern and southern hemispheresfrom any chance of intermixture. This one barrier would bemore effectual than it is now, since there would be no loftymountain ranges to serve as a bridge for the partial interchangeof northern and southern forms.3. If such a condition of the earth as here supposed continuedfor very long periods, we may conceive that the actionand reaction of tjie various organisms on each other, combinedwith the influence of very slowly changing physical conditions,would result in an almost perfect organic balance, which wouldbe manifested by a great stability in the average numbers, thelocal range, and the peculiar characteristics of every species.4. Under such a condition of things it is not improbable thatthe total number of clearly differentiated specific forms might bemuch greater than it is now, though the number of generic andfamily types might perhaps be less ;for dominant species wouldhave had ample time to spread into every locality where theycould exist, and would then become everywhere modified intoforms best suited to the permanent local conditions.5. Now let us consider what would be the probable effect ofthe introduction of a barrier, cutting off a portion of this homogeneousand well-balanced world. Suppose, for instance, that asubsidence took place, cutting off by a wide arm of the sea alarge and tolerably varied island. The first and most obviousresult would be that the individuals of a number of specieswould be divided into two portions, while others, the limits ofwhose range agreed approximately with the line of subsidence,would exist in unimpaired numbers on the new island or on themain land. But the species whose numbers were diminishedand whose original area was also absolutely diminished by theportion now under the sea, would not be able to hold their