The final quarter of the nineteenth century marked
a turning point in the history of biology—biologists
became less interested in applying an ideal of
historical explanation deductively to organic function and more interested in discerning the causes of vital
processes through experimental manipulation. But it
is impossible to discuss the history of biology in the
nineteenth century without emphasizing that those
areas of biology most in the public eye had depended
on historical explanation. Wherever it was applied,
historical explanation was deemed causal explanation.
The biologist-as-historian and the general historian of
human events dealt with comparable phenomena and
assumed necessarily a common mode of explanation.
   Nineteenth-century biologists found a historical
explanation of organic function attractive partly
because their observation of the formation of a
new cell from a preexisting cell seemed to confirm
a historical explanation of cell generation. The
same direct observation of continuous stages of
development was also possible when they examined
the complex sequence of events of embryogenesis.
In both cases, the observer received a concrete
impression that the daughter cell was brought into
being, or caused, by the prior cell. The argument
that these scientists employed confuses temporal
succession and causal explanation, of course,
but such confusion is the heart of most historical
explanation.
   Not surprisingly, the evolutionary biologists of
the nineteenth century encountered a particularly
troublesome problem in their attempts to document
historical explanation convincingly: the factual record
of the history of life on earth (e.g., that provided by
fossils) was incomplete. The temporal continuity of
living forms was convincing, but was an assumption
that was difficult to uphold when one compared
species or organisms forming any two stages of the
evolutionary record. Nineteenth-century biologists
recognized this problem and attempted to resolve
it. Their solution today appears to be only verbal,
but was then regarded as eminently causal. The fact
of evolution demanded some connection between
all reproducing individuals and the species that they
compose, as well as between living species and
their extinct ancestors. Their solution, the concept
of heredity, seemed to fill in an admittedly deficient
historical record and seemed to complete the
argument for a historical explanation of evolutionary events.