In the first place, like a living organism, truth grows, and its gradual evolution may be traced from the tiny germ to the mature product.

Never springing, Minerva-like, to full stature at once, truth may suffer all the hazards incident to generation and gestation.

Much of history is a record of the mishaps of truths which have struggled to the birth, only to die or else to wither in premature decay.

Or the germ may be dormant for centuries, awaiting the fullness of time.
Secondly, all scientific truth is conditioned by the state of knowledge at the time of its announcement.

Thus, at the beginning of the seventeenth century, the science of optics and mechanical appliances had not made possible (so far as the human mind was concerned) the existence of blood capillaries and blood corpuscles.