INTRODUCTION. XXI 



entities so simple and minute, it is difficult to detect any very 

 manifest specialties of function, or to assign to them the attri- 

 butes that are obviously characteristic of higher organisms ; 

 but if we can show that their organization ministers to func- 

 tions that are purely of a vegetative kind, and that none of the 

 circumstances which attend their growth, development, and re- 

 production, are inconsistent with the known phenomena of 

 plant-life, we are warranted, and indeed constrained, to associate 

 them in a systematic arrangement with that department of 

 organized being to which they are thus functionally and struc- 

 turally allied, or from which at all events they are not in these 

 respects excluded by any well-ascertained or notable diversities. 

 We conclude then that the Diatomacese are plants belonging 

 to the Sub-Class Algse ; or, following the more recent system- 

 atists, to the Class Protophyta, all of whose forms, unless when 

 united into filaments, or aggregated into masses by the mecha- 

 nical aid of the mucus they so frequently secrete, are micro- 

 scopic and unicellular, — homologues of the component parts of 

 the tissue which forms the entirety of the Thallogen, and enters 

 largely into the composition of the higher and more organized 

 forms of vegetable life. The Diatomacese, with specialties of 

 their own, have also intimate alliances with the other orders of 

 the Protophyta, resembling the Zygnemacecz and Desmidiaceoe in 

 the reproductive process, — the Nostocacece in the tendency shown 

 by several genera to surround their frustules with frondose 

 masses of mucus, within which linear series of cells are subse- 

 quently developed, — the Oscittatoriacece in their movements, — 

 the Pahnettacece and all the orders I have named, in the self- 

 dividing act by which the individuals of the species are multi- 

 plied, or the aggregate of specific life maintained and increased. 



Section X. 



On the Determination op Species in the Diatomace^;. 



In an order prolific of forms, so minute in size, and simple in 

 organization, it is by no means an easy matter to fix upon any 

 certain elements of specific arrangement. 



