OF ORQANIG BEINGS. 347 



are found, the line will sometimes falsely appear to begin 

 at its lower end, not in a sharp point, but abruptly; it 

 then gradually thickens upward, often keeping of equal 

 thickness for a space, and ultimately thins out in the 

 upper beds, marking the decrease and final extinction of 

 the species. This gradual increase in number of the species 

 of a group is strictly conformable with the theory, for the 

 species of the same genus, and the genera of the same 

 family, can increase only slowly and progressively; the 

 process of modification and the production of a number of 

 allied forms necessarily being a slow and gradual process, 

 one species first giving rise to two or three varieties, these 

 being slowly converted into species, which in their turn 

 produce by equally slow steps other varieties and species, 

 and so on, like the branching of a great tree from a single 

 stem, till the group becomes large. 



ON BXTiiircTiosr. 



"We have as yet only spoken incidentally of the disap- 

 pearance of species and of groups of species. On the 

 theory of natural selection, the extinction of old forms 

 and the production of new and improved forms are inti- 

 mately connected together. The old notion of all the 

 inhabitants of the earth having been swept away by catas- 

 trophes at successive periods is very generally given up, even 

 by those geologists, as Elie de Beaumont, Murchison, Bar- 

 rande, etc., whose general views would naturally lead 

 them to this conclusion. On the contrary, we have every 

 reason to believe, from the study of the tertiary formations, 

 that species and groups of species gradually disappear, one 

 after another, first from one spot, then from another, and 

 finally from the world. In some few cases, however, as by 

 the breaking of an isthmus and the consequent irruption 

 of a multitude of new inhabitants into an adjoining sea, 

 or by the final subsidence of an island, the process of ex- 

 tinction may have been rapid. Both single species and 

 whole groups of species last for very unequal periods; some 

 groups, as we have seen, have endured from the earliest 

 known dawn of life to the present day; some have disap- 

 peared before the close of the palseozoio period. No fixed 

 law seems to determine the length of time during which 



