Three of the most useful working concepts on this site are Pangenesis, Epigenetics, and Vitalism. One was proposed and largely abandoned; one is mainstream science today; one has refused to die and keeps returning to the conversation under different names. What follows are short working definitions.
Pangenesis
In 1868, Charles Darwin proposed Pangenesis, a developmental theory of heredity. He suggested that all cells in an organism are capable of shedding minute particles he called gemmules, which circulate throughout the body and finally congregate in the gonads. These particles are then transmitted to the next generation and are responsible for the transmission of characteristics from parent to offspring. If any cells of the parent undergo changes as a result of environmental change, they will consequently transmit modified gemmules to their offspring.
Soon after Darwin's pangenetic theory was published, Francis Galton designed a series of blood transfusion experiments on differently pigmented rabbits to test its validity. He found no evidence in support of the existence of Darwin's gemmules, and the concept of Pangenesis was largely abandoned. Interestingly, certain elements of the picture Darwin painted — the idea that cells communicate information about environmental stress upward in a way that can affect inheritance — have returned, in altered form, in the modern study of epigenetics.
Epigenetics
In the science of genetics, epigenetics is the study of cellular and physiological phenotypic-trait variations that result from external or environmental factors that switch genes on and off and affect how cells express genes. Epigenetic research seeks to describe dynamic alterations in the transcriptional potential of a cell. These alterations may or may not be heritable, although the use of the term epigenetic to describe processes that are not heritable is controversial.
Unlike genetics based on changes to the DNA sequence (the genotype), the changes in gene expression or cellular phenotype of epigenetics have other causes — hence the prefix epi- (Greek επί: over, outside of, around).
Vitalism
Vitalism is a theory holding that the characteristics and behaviors of living things are expressed through a vital force and not by biophysical means alone. Vitalism has a long history in medical philosophies, with ties to the four humors. It is sometimes referred to as a “life spark,” or as the soul itself. In the Eastern traditions it is essentially the same idea as qi or chi, which is woven deeply into Oriental medical methods.
Modern molecular biology has, of course, done remarkable work without invoking vitalism. Yet questions about the difference between an organism that is alive and the same arrangement of matter immediately after death remain interesting, and any complete account of inheritance has to say something about what is being inherited beyond a sequence of bases.
A work in progress, to be continued.