The meaning of a sentence.Thanks. Context： Great as was the public concern which followed the triumphant announcement of the first nation to establish a satellite weapon satisfactorily, a still greater concern was felt over the failure of others to make any announcement at all, even when they were known to have had similar successes. It was by no means pleasant to realize that there was an unknown number of menaces up there over your head, quietly circling and circling until someone should arrange for them to drop and that there was nothing to be done about them. Still, life has to go on and novelty is a wonderfully short- lived thing. One became used to the idea perforce. From time to time there would be a panicky flare-up of expostulation when reports circulated that as well as satellites with atomic heads there were others with such things as crop diseases, cattle diseases, radioactive dusts, viruses, and infections not only of familiar kinds but brand-new sorts recently thought up in laboratories, all floating around up there. Whether such uncertain and potentially backfiring weapons had actually been placed is hard to say. But then the limits of folly itself particularly of folly with fear on its heels are not easy to define, either. A virulent organism, unstable enough to become harmless in the course of a few days (and who is to say that such could not be bred?), could be considered to have strategic uses if dropped in suitable spots. ====================== So，what‘s the meaning of “But then the limits of folly itself particularly of folly with fear on its heels are not easy to define, either.” Thank you.