Without going into any details, is not this a strong general argument?
1.

A species varies occasionally in two directions, but owing to their free intercrossing the varieties never increase.
2.

A change of conditions occurs which threatens the existence of the species; but the two varieties are adapted to the changing conditions, and if accumulated will form two new species adapted to the new conditions.
3.

Free crossing, however, renders this impossible, and so the species is in danger of extinction.
4.

If sterility would be induced, then the pure races would increase more rapidly, and replace the old species.
5.