4 INTRODUCTORY LECTURE. 



coarsPT form, passing by the more obvious mouth into the wider 

 digestive sac of the higher animals. There is no essential or well- 

 defined distinction of assimilative structure here : the diflPerence is at 

 most one of form and proportion of the internal cavities and of their 

 external openings : they are the same as to function in plants and 

 animals. The more free and locomotive the organism, the more 

 capacious the internal receptacle for the matters to be assimilated, 

 the characteristic differences of form fading away in the passage from 

 the pendant parasites and the polypes to the astomatous polygastria, 

 the sponges, and plants proper. So that if the presence of absorbent 

 pores and assimilative cells, instead of a mouth and stomach, be 

 deemed a Vegetable chai'acteristic, then this, like the rooted character, 

 mounts up a certain way into the Animal Kingdom. 



When the chemist entered upon his valued investigations into the 

 changes which living organisms wrought upon surrounding media 

 and those media upon them, he found that plants and animals 

 differed in their behaviour in these respects. Plants exhaled oxygen, 

 animals carbonic acid : the gas which was food to plants was poison 

 to animals ; animals inspired the oxygen which plants exhaled. 

 Thus the balance of the gases in the atmosphere was beneficially 

 maintained by the antagonistic actions of the two kingdoms. And 

 in a general way this is true, but the chemical antagonism fails as a 

 boundary line where we most require it, as we approach, viz., the 

 confines of the two kingdoms. Wohler * has shown that some of the 

 free and locomotive Polygastria, e. g., Chlamidotnonas pulvisculus, 

 Euglena viridis, Frustulia salina, eliminate pure oxygen, as the 

 ultimate metamorphosis of their tissues : and on the other hand, 

 Drs. Schlossberger and Dopping f have proved that mushrooms and 

 sponges exhale carbonic acid. The green-coloured matter called 

 " chlorophyll " which is common in most plants, exists in the Poly- 

 gastria, in the green Planarite, and the fresh-water polype. 



As regards the conversion of the surrounding elements into their 

 own matter, animals combine carbon, hydrogen, oxygen, and nitrogen 

 to form the proximate principles of most of their tissues, which are 

 thus " quaternary " compounds ; whilst the tissues of plants are in 

 general " ternary " compounds of carbon, hydx'ogen, and oxygen, and 

 sometimes, as, e. g., the cellulose substance, only " binary " ones of 

 carbon and hydrogen. But if the presence of nitrogen in the 

 organic tissues be taken as an animal characteristic, then we find it 

 descending pretty far into the vegetable series, the element being 



* VIII. 1843 p. 206. -f lb. bd. Hi. p. 119. 



