Extracts to show Darwin's views about the evolution of the sex ratio.

Why was Darwin interested in the sex ratio?   Because it would be simple to explain why males were more elaborated if females were in shorter supply.  However, after reviewing the evidence, Darwin deduced that sex ratios were usually about 1:1. See also Darwin on why males are more passionate.

But after investigating, as far as possible, the numerical proportion of the sexes, I do not believe that any great inequality in number commonly exists.

...

Numerical Proportion of the Two Sexes.- I have remarked that sexual selection would be a simple affair if the males were considerably more numerous than the females. Hence I was led to investigate, as far as I could, the proportions between the two sexes of as many animals as possible; but the materials are scanty. I will here give only a brief abstract of the results, retaining the details for a supplementary discussion, so as not to interfere with the course of my argument.

...

The proportion of the sexes in relation to natural selection
...
I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution for the future.