2 REVIEWS. 



in a species of Diatomacese a process entirely similar to the " conjugation" 

 with which botanists were already familiar, as occurring in several of the 

 green freshwater algae. In Enotia turgida, in which he first witnessed 

 this phenomenon, two of the microscopic unicellular plants, of which this 

 species consists, were seen to approach one another, and pour out from 

 within their siliceous skins, which opened for the purpose, the whole of the 

 soft, semi-fluid contents of their bodies, mingling the one with the other, 

 and leaving behind nothing but the empty shells. The mingled contents 

 soon shaped themselves into two cylindrical masses, or " sporangia," which 

 gradually became more and more similar to the parents, encasing them- 

 selves, like them, in a flinty shell, and ultimately differing from them chiefly 

 in their very much larger size. 



We well remember hearing Mr. Thwaites's paper read for the first time, 

 at one of the meetings of the British Association ; and the interest which 

 the botanists then present took in so important an announcement will not 

 easily be forgotten ; for it was deemed that the point was at last discovered 

 which decided the true relations of these enigmatical organisms ; that 

 their supposed animality was a simple delusion ; and that the territory of the 

 Diatom must, without further demur, be handed over to the dominion of 

 Flora. 



With the exception of Mr. Thwaites's subsequent statement of some addi- 

 tional instances of the process just described, and his discovery of the 

 interesting fact that in certain species the " sporangium" repeats itself by 

 division, our knowledge of the conjugation of the Diatomaceas had rested 

 pretty much as he left it. It was still evident, however, that something 

 yet remained to be determined, and the true import of the sporangial frus- 

 tule, the immediate result of the curious conjugation, continued unex- 

 plained. 



When a subject like the present falls into the hands of a patient and 

 laborious observer, we may safely expect some results ; and it cannot, 

 therefore, be matter of surprise that Mr. Smith is enabled, in the volume 

 before us, to present us with numerous additional instances of conjugation, as 

 well as confirmations of the very accurate accounts of Thwaites, making 

 in all no less than twenty-seven distinct cases, which have all come under 

 his own observation. 



But a still more important step towards the determination of the true 

 significance of Conjugation, than that suggested by the mere accumulation 

 of instances, has been made by Mr. Smith. The ultimate destination of the 

 " sporangial frustule," the immediate product of the conjugative act, had 



