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.
The author implies that nineteenth-century biologists who studied embryogenesis believed that they
had discovered physical evidence that supported their use of historical explanation
were the first biologists to call for systematic experimentation on living organisms
were able to use historical explanation more systematically than were biologists who did not study embryogenesis
had inadvertently discovered an important part of the factual record of the history of living organisms on earth
had avoided the logical fallacies that characterize the reasoning of most nineteenth-century biologists