Answer:
By his fame.
Step-by-step explanation:
The speaker starts their text by presenting the life of a character, which is destined for "Fortune and to Fame unknown", meaning that's the medium from which the quality of the character's life in the future will be measured.
The mention of science is used to describe the character's birth as a natural occurrence, and melancholy represents the distrust of the speaker about the quality of this new life.