When the tropical species retreated as far as they could to the equator they would halt, and then the confusion would spread back in the line of march from the far north, and the strongest would struggle forward, etc., etc.

(But I am getting quite poetical in my wriggles).

In short, I THINK the warm-temperates would be exposed very much longer to those causes which I believe are alone efficient in producing change than the sub-arctic; but I must think more over this, and have a good wriggle.

I cannot quite agree with your proposition that because the sub-arctic have to travel twice as far they would be more liable to change.

Look at the two journeys which the arctics have had from N.to S.and S.to N., with no change, as may be inferred, if my doctrine is correct, from similarity of arctic species in America and Europe and in the Alps.