He gets right back to the heart of primal things.

When he wrote that line he was not really thinking that there was a nasty poison in the heart of a woman or death in her hands.

What he was thinking was that in the jungle the female lion or tiger or jaguar must go and find a particularly secluded cave and bear her young and raise them to be quite active kittens before she leads them out, because there is danger of the bloodthirsty father eating them when they are tiny and helpless.

And if perchance a male finds the cave of his mate and her tiny young and enters it to do mischief, then there is no recorded instance I know of in which the female, fighting in defense of her young, has not been 'more deadly than the male.' And that is the origin of the much-discussed line concerning the female of the species, and it holds good fairly well down the line of the wild.

It's even true among such tiny things as guinea pigs and canary birds.