704 



before we allow the frond of a fern greater iraportance in the economy 

 of the plant than it really possesses. Three years ago I attempted to 

 introduce Polypodium vulgare on a wall at Peckham, the fronds 

 withered and fell, but now, since the recent rains in October, and long 

 after I supposed the plant dead, two fronds have appeared, and though 

 small, they will probably live through the winter : thus it will appear evi- 

 dent, not only that the fronds are not the plant, but that the plant can 

 exist for two years without their assistance. I therefore incline to re- 

 gard the rhizoma as the plant in Polypodium vulgare, and its fronds 

 as respiratory and reproductive appendages, which, when their office is 

 performed, wither and fall, without^ injury to the plant which bears 

 them : this idea will I think receive additional confirmation from an 

 attentive examination of the bulky rhizoma of Davallia Canariensis. 

 Now if this bulky rhizoma is the plant, as the slug is certainly the 

 animal, it is the limaciform rhizoma, and not the beautifully divided 

 frond of the Davallia, that we must compare with the slug : the same 

 may be said of Polypodium and Trichomanes : their vermiform rhizo- 

 mata may fairly be conipared with Annelides, without any extraordinary 

 strain on the inventive faculties. Pursuing this subject still further, we 

 shall find that many limaciform nudibranchiate mollusks throw out, as 

 respiratory organs, beautifully branched fronds resembling those of 

 ferns.* In reproduction there is also a correspondence : mollusks are 

 hermaphrodite, each individual is productive, and ferns produce seed 

 without the apparent presence of stamens, or anything to which 

 can be assigned a similar office ; so that the sexes are mixed, and 

 not distinct as in the vertebrates and exogens, and the articulates and- 

 endogens. I believe the more the subject is investigated, the more 

 strict will be found the analogy between mollusks among animals, and 

 acrogens among vegetables. 



We thus arrive at the following parallel 



Plans of Structure. 

 In the Animal Kingdom. In the Vegetable Kingdom. 



Vertebrate -= Exogen 



Articulate __ Endogen 



Molluscate = Acrogen 



Radiate __ Thallogen 



The next proposition relates to the position of these divisions. 

 I have taken great pains to show that the four divisions of the Ani- 



* Tritonia arborescens of Cuvier is a remarkable instance of this ; it has seven or 

 eight pairs of these branchial fronds, those towards the head being the largest and 

 those nearest the tail the smallest : the anterior pairs are beautifully divided and sub- 

 divided, like the fronds of Davallia. 



