I do not believe that the amount of food by any means is the sole determining cause of number.
Lessened fertility is equivalent to a new source of destruction.

I believe if in one district a species produced from any cause fewer young, the deficiency would be supplied from surrounding districts.

This applies to your Paragraph 5.

(213/1.

See Letter 211.) If the species produced fewer young from any cause in every district, it would become extinct unless its fertility were augmented through Natural Selection (see H.Spencer).
I demur to probability and almost to possibility of Paragraph 1., as you start with two forms within the same area, which are not mutually sterile, and which yet have supplanted the parent-form.
(Paragraph 6.) I know of no ghost of a fact supporting belief that disinclination to cross accompanies sterility.