In 1853 a celebrated geologist, Count Keyserling ( Bulletin de la Soc. Géolog. , 2nd Ser. , tom. x , page 357), suggested that as new diseases, supposed to have been caused by some miasma have arisen and spread over the world, so at certain periods the germs of existing species may have been chemically affected by circumambient molecules of a particular nature, and thus have given rise to new forms.