316 
GEOLOGICAL SUCCESSION. 
Chap. X. 
credible that a fantail, identical with the existing breed, 
could be raised from any other species of pigeon, or 
even from the other well-established races of the do¬ 
mestic pigeon, for the newly-formed faintail would be 
almost sure to inherit from its new progenitor some 
slight characteristic differences. 
Groups of species, that is, genera and families, follow 
the same general rules in their appearance and disap¬ 
pearance as do single species, changing more or less 
quickly, and in a greater or lesser degree. A group 
does not reappear after it has once disappeared; or 
its existence, as long as it lasts, is continuous. I am 
aware that there are some apparent exceptions to this 
rule, but the exceptions are surprisingly few, so few 
that E. Forbes, Pictet, and Woodward (though all 
strongly opposed to such views as I maintain) admit 
its truth; and the rule strictly accords with my theory. 
For as all the species of the same group have descended 
from some one species, it is clear that as long as any 
species of the group have appeared in the long succession 
of ages, so long must its members have continuously 
existed, in order to have generated either new and 
modified or the same old and unmodified forms. Species 
of the genus Lingula, for instance, must have continu¬ 
ously existed by an unbroken succession of generations, 
from the lowest Silurian stratum to the present day. 
We have seen in the last chapter that the species 
of a group sometimes falsely appear to have come in 
abruptly; and I have attempted to give an explana¬ 
tion of this fact, which if true would have been fatal 
to my views. But such cases are certainly excep¬ 
tional; the general rule being a gradual increase in 
number, till the group reaches its maximum, and 
then, sooner or later, it gradually decreases. If the 
number of the species of a genus, or the number of 
