XX INTRODUCTION. 



the animal kingdom, for the secretion of siliceous spicula, as 

 an internal skeleton, in some of the Spongidese, cannot be re- 

 garded as an analogous phenomenon ; whereas the vegetable 

 kingdom furnishes us with cases, not merely of the secretion of 

 silex as a vegetable product in the Bamboo, but with frequent 

 instances of its intimate union with cellulose in the membrane 

 which forms the epiderm of the cell, as in the Natural Orders 

 already mentioned, in the Palmaceae and others. 



The rapid evolution of oxygen from the frustules of the Diato- 

 macese while in the active discharge of their nutritive function, 

 under the influence of the sun's light and heat, is another cir- 

 cumstance which confirms the view we have taken of their vege- 

 table nature : this may be noticed in any mass of Diatomacese 

 during the warmer months of the year, or in gatherings freely 

 exposed to the sun, in the elevated temperature of a confined 

 apartment during the winter or spring. Under these condi- 

 tions the water in the vessel becomes covered with minute 

 bubbles of oxygen, and portions of the Diatomaceous stratum 

 are floated up by the buoyancy of the globules of this gas ad- 

 hering to their frustules. Such phaenomena can only be ac- 

 counted for by supposing that the Diatomacese are plants, and 

 that they exhale, like all plants in a state of active vegetation, 

 oxygen from their tissues ; but this process is irreconcileable 

 with the hypothesis of their animal nature. 



Another view of the nature of the Diatomacese has been 

 advocated by some writers, who, admitting that the arguments 

 for the animality of these organisms are devoid of weight, main- 

 tain that the evidence of their vegetable nature is equally incon- 

 clusive, and contend that they occupy a neutral position between 

 the two kingdoms, and cannot in the present state of our know- 

 ledge be assigned to either. It does not appear to me that any 

 benefit would accrue to physiology or science by thus consti- 

 tuting a new domain of being, occupied by organisms of inde- 

 finable and mysterious attributes, and for whose description, if 

 description in their case were possible, it would be necessary to 

 invent a new and probably unintelligible nomenclature. This 

 would be merely to disguise our ignorance under the veil of a 

 metaphysical sophistry. It may be admitted, that in the case of 



