Final answer:
The theory stating that a fully formed miniature human is pre-existing in the father's sperm is known as the preformation theory. This concept was later disproved through modern genetics and understanding of mitosis.
Step-by-step explanation:
The statement that "The whole baby (organism or human) was already preformed in the father's sperm, plus each preformed body also had an infinite number of other preformed bodies within it" is known as the preformation theory. This theory, which dates back to the early microscopic observations by Anton von Leeuwenhoek, suggested that a fully formed miniature human, a homunculus, existed inside the sperm and only needed to grow in size. However, this idea was disproved through the understanding of genetics, cell division, and embryonic development. We now know that a zygote, the single cell formed by the union of a sperm and an egg, contains genetic material from both parents. Through a process called mitosis, the zygote divides repeatedly to form trillions of cells that eventually differentiate to become the complex structures seen in a fully formed infant.