The biblical myth of preformationism was strongly favored as the dominant Christian theology. Epigenesis instead suggested that unorganized organic matter could somehow generate life without the need for God’s intervention. How dare it.
The microscope brought forth a religious quandary, centered on sperm (shades of Aristotle). Observing sperm as a life form, how could it be that so many little animals were wasted by even a single ejaculation?
In the early 18th century, German philosopher Gottfried Leibniz gave spermism a lift with the notion of panspermism: that wasted sperm could be scattered by the wind and generate life upon finding a suitable host. Leibniz also believed that “death is only a transformation enveloped through diminution.” For Leibniz, not only have organisms always existed in their living form, they will always exist, body united with soul, in an endless cycle of death and rebirth.
English physician William Harvey was first in the Western world to write of the circulatory system: of blood being pumped around the body by the heart. Harvey’s 1651 On the Generation of Animals pumped epigenesis, contradicting Aristotle’s conjecture of preformationism (in his identically titled treatise).
Harvey famously asserted ex ovo omnia: all animals come from eggs. This was misunderstood in some biased quarters as suggesting ovist preformationism, egg-based preformationism, but Harvey disclosed epigenesis as the basis for biological development.
As gametes were too small to be seen under the best magnification at the time, Harvey’s account of fertilization was speculative rather than descriptive. Early on, Harvey proposed that a “spiritous substance” fertilized a female, but he later spurned it as superfluous, and so unscientific.
Harvey’s 2nd guess was that fertilization transpired through a mysterious transference, either by contact or contagion – contact indeed, of a most contagious sort.
Preformationism clung in quarters given to Christianity for some time. In the late 17th century, French philosopher Nicolas Malebranche sought to reconcile the teachings of Augustine of Hippo and Descartes, to prove that God was omnipresently active. Malebranche proposed that each embryo held ever smaller embryos ad infinitum: a biological Matryoshka doll.
Preformationism dominated in the 18th century, competing with spontaneous generation and epigenesis, as those theories held that inert matter could create life without God’s intervention. It wasn’t until the late 19th century that cell theory would succeed preformationism as the accepted model of life.