270 STUDIES, SCIENTIFIC AND SOCIAL 



at last come when two or three species only will remain, 

 most likely in widely separated parts of its former area ; 

 their position being determined either by the competition 

 being there somewhat less severe, or by some speciality 

 of conditions which are exceptionally favourable to the 

 dying-out group. Then one and then another of these 

 species will die out, and the once extensive genus will only 

 be represented by a single species inhabiting a very 

 restricted locality. This will become rarer and rarer, the 

 necessary preliminary to that final extinction which we 

 know to be the fate, sooner or later, of every group of 

 living things. 



Most working naturalists (and none better than Dr. 

 Sclater) are acquainted with genera whose distribution 

 will illustrate all the successive phases of this hypothetical 

 history; while palaeontology furnishes us with some 

 actual examples of the progress of a group from its rise 

 to its decay, though, owing to the extreme imperfection 

 of the geological record (and its total absence for 

 important epochs in many parts of the globe), we can 

 never trace the complete history of such a group. A 

 little consideration will show us, however, why it is that 

 continuity of generic and specific areas appears to be the 

 rule, discontinuity the exception, although the reverse 

 may really be the case. There can be no doubt that the 

 development of an extensive genus is a slow process, 

 while its decay and final extinction need not be slow, and 

 may conceivably be extremely rapid. Geological and 

 geographical changes may be long in preparation, but 

 finally very abrupt. Land may sink a thousand feet with- 

 out producing any very important effect except diminution 

 of area, but the next hundred feet of depression may cut 

 it off from a continent, and may alter the direction of 

 ocean currents, thus producing a greater organic and 

 physical change than had been brought about by the 

 previous subsidence occupying ten times as long. Again, 

 such a change as that which admitted the highly 

 organised Miocene mammalia of Europe into Tropical and 

 South Africa must have led at once to the extermination 

 of many of the indigenous species, and have restricted the 



