In presenting this little monograph to the profession, I have no apologies to offer. My only hope is, that the subject may be as seriously considered as it deserves. Believing that a superabundance of modesty has prevented facts from being duly presented to the profession, and thinking myself endowed with less of that desirable virtue than others, I can but conclude that my reputation will not suffer from reverses such as might ensue from elucidating theories, and maybe fancies, connected with such a delicate topic. But if professional minds are stimulated to a renewed study of these important phenomena, my highest anticipations will be reached.