XX11 INTRODUCTION. 



The determination of genera being mainly an artificial mode 

 of aiding research, or conveniently grouping together forms 

 possessing in common important and obvious characteristics of 

 structure or of function, must, to a great extent, be influenced 

 by the theoretic views, and be left to the experience or judge- 

 ment of the individual inquirer ; but it is far otherwise with the 

 determination of species. We here seek to discover the di- 

 stinctions which have been impressed by nature upon every 

 individual derived by reproduction or by self-division from the 

 original product of the creative act. Such distinctions may, to 

 our powers of apprehension or discovery, be of the slightest 

 kind, and may, in the marvellous minuteness and multiplicity of 

 these organisms, blend into one another by gradations too fine 

 to be detected by any appliances that we can employ. In an 

 attempt to accomplish this object, it is a point of the first 

 importance to select peculiarities of structure or organization 

 which in their main features are common to every species in 

 each genus we may adopt, and yet exhibit variations in each 

 species of such genus. If several such peculiarities can be 

 found, our task would be an easy one ; but the simplicity of 

 organization in the DiatomaceEe forbids this expectation, and 

 usually reduces us within narrower limits. I shall lay before 

 the inquirer the results of my own experience on this subject, 

 without claiming for the conclusions to which I have arrived 

 any absolute authority, feeling assured that a far wider know- 

 ledge of these forms than that to which I can at present pretend, 

 would be necessary to give to such conclusions the weight of 

 established truth. 



We have seen that the ordinary Diatomaceous frustule owes 

 its reproduction to the protoplasmic contents of the sporangial 

 frustule formed during the process of conjugation. 



The embryonic frustules which are generated within the 

 sporangial cyst, having, by their increase in size, burst the mem- 

 brane which contains them, escape from the cyst, and in a 

 definite, but unascertained period, reach the mature form and 

 size of the ordinary frustule. 



The further growth and modification in form of the individual 

 cells seem now to be arrested bv the consolidation of the sili- 



