why couldn't victor frankenstein bring another monster to life ?

In Chapter Twenty, Frankenstein contemplates the possible effects of a second experiment. He becomes increasingly horrified by his task and finds himself tormented by a number of questions: will this second creature be even more malignant than the first? Will she, unlike her mate, refuse to quit the company of man? Will they ultimately despise each other's hideousness as a mirror of their own? Frankenstein is repulsed by the thought that the two monsters might beget children, thereby creating a new race that could ultimately destroy all humanity. Victor decides that unleashing such a scourge upon mankind would be of the utmost selfishness.