xxiv HISTOBICAL SKETCH. 



In 1853 a celebrated geologist, Count Keyserling 

 (' Bulletin de la Soc. G6olog.,' 2nd Ser., torn. x. p. 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 circum- 

 ambient molecules of a particular nature, and thus have 

 given rise to new forms. 



In this same year, 1853, Dr. Schaaffhausen published 

 an excellent pamphlet (' Verhand. des Naturhist. Vereins 

 der Preuss. Eheinlands,' &c.), in which he maintains 

 the development of organic forms on the earth. He 

 infers that many species have kept true for long periods, 

 whereas a few have become modified. The distinction 

 of species he explains by the destruction of intermediate 

 graduated forms. " Thus living plants and animals are 

 not separated from the extinct by new creations, but 

 are to be regarded as their descendants through contin- 

 ued reproduction." 



A well-known French botanist, M. Lecoq, writes in 

 1854 ('Etudes sur Geograph. Bot.,' tom. i. p. 350), 

 " On voit que nos recherches sur la fixite ou la variation 

 de I'espece, nous conduisent directement aux idees 



palaeontologist Unger published, in 1852, his belief that species 

 undergo development and modification. Dalton, likewise, in 

 Pander and Dalton's work on Fossil Sloths, expressed, in 1821, a 

 similar belief. Similar views have, as is well known, been main- 

 tained by Oken in his mystical ' Natur-Philosophie.' Prom other 

 references in Godron's work 'Sur I'Bspece,' it seems that Bory 

 St. Vincent, Burdach, Poiret, and Fries, have all admitted that new 

 species are continually being produced. 



I may add, that of the thirty-four authors named in this 

 Historical Sketch, who believe in the modification of species, or at 

 least disbelieve in separate acts of creation, twenty-seven have 

 written on special branches of natural history or geology. 



