Distinction between the sterility of first crosses and of hybrids-
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication-Laws governing the
sterility of hybrids Sterility not a special endowment, but
incidental on other differences, not accumulated by natural
selection-Causes of the sterility of first crosses and of hybrids
-Parallelism between the effects of changed conditions of
life and of crossing-Dimorphism and Trimorphism-Fertility
of varieties when crossed, and of their mongrel offspring not
universal-Hybrids and mongrels compared independently of
their fertility-Summary
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day-On
the nature of extinct intermediate varieties; on their number
-On the lapse of time, as inferred from the rate of denudation
and of deposition-On the lapse of time as estimated by years
-On the poorness of our paleontological collections-On the
intermittence of geological formations-On the denudation of
granitic areas-On the absence of intermediate varieties in any
one formation-On the sudden appearance of groups of species
-On their sudden appearance in the lowest known fossiliferous
strata-Antiquity of the habitable earth