Chap. X. 
EXTINCTIOK. 
317 
the genera of a family, be represented by a vertical 
line of varying thickness, crossing the successive geo¬ 
logical formations in which the species are found, the 
line will sometimes falsely appear to begin at its lower 
end, not in a sharp point, but abruptly; it then gradu¬ 
ally thickens upwards, sometimes keeping for a space 
of equal thickness, 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 my 
theory; as the species of the same genus, and the 
genera of the same family, can increase only slowly and 
progressively; for the process of modification and the 
production of a number of allied forms must be slow 
and gradual,—one species giving rise first to two or 
three varieties, these being slowly converted into species, 
which in their turn produce by equally slow steps other 
species, and so on, like the branching of a great tree 
from a single stem, till the group becomes large. 
On Extinction ,—^We have as yet spoken only inci¬ 
dentally of the disappearance of species and of groups 
of species. On the theory of natural selection the ex¬ 
tinction of old forms and the production of new and im¬ 
proved forms are intimately connected together. The 
old notion of all the inhabitants of the earth having 
been swept away at successive periods by catastrophes, 
is very generally given up, even by those geologists, 
as Elie de Beaumont, Murchison, Barrande, &c., whose 
general views would naturally lead them to this conclu¬ 
sion. 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. Both single species and whole 
