22 GENERAL HISTORY OF 



from their extreme minuteness, which, even with the 

 assistance of our most perfect microscopes, places them at 

 the very limit of our vision. The great importance of this 

 subject, especially to the medical professor, has obtained 

 for it, from several distinguished naturalists, long and 

 laborious researches ; but, on the whole, the results have 

 been so contradictory, as by no means to justify the intro- 

 duction of them into this manual. It will be sufficient, 

 therefore, to say, that since the time of their discovery 

 (1676), up to the present period, all that we know of the 

 true Spermatozoa of animals, is, that they are not distin- 

 guishable from the Cercaria found in the liver of snails, 

 the animal organization of which has been made out by 

 Bauer_, Wagner, and Ehrenberg. 



The recent discoveries of Dr. Unger on the spermatozoa 

 of plants is a subject of such deep interest, and so little 

 known in this country, that I have introduced a description 

 of them under the genus Spirillum ; while original draw- 

 ings of them will be found in Plate XII. 



It has been said that the hne of demarcation between many 

 species of animals and plants — the transition from the 

 one kingdom to the other — is not easily defined. Indeed, 

 so close is the connection between them, that some mem- 

 bers of the families Closterina, Vibrionia, and Bacillaria, 

 which are considered by Ehrenberg to be animals, are, by 

 many eminent botanists, set down as belonging to the 

 vegetable kingdom, and classed with the minute aquatic 

 algae of the genera Oscillatoria, Spyrogyra, &c. The true 

 species of the two genera just named, it must be admitted, 

 are not of animal structure ; and Dr. Ehrenberg has given 

 us the following reasons why they are not included with 



