SYSTEMATIC ACCOUNT. 87 



being subject to much variation of form, due probably 

 to a great extent to external causes, which however 

 are not always apparent. Though it is a matter of 

 convenience to distinguish extreme forms by varietal 

 names, they are, as a rule, connected with each other 

 by an unbroken series of intermediates. Some authors 

 have attempted to give names to every slight variation. 

 In this way Dr. Migula has enumerated with elabo- 

 rate diagnoses no less than 70 European varieties 

 of "G. fcetida (G. vulgaris). While admiring such 

 patience and assiduity, it is difficult to see that any 

 useful end is served by the attempt to make minute 

 distinctions between plants which are evidently mere 

 states, besides which there is no limit to such 

 differentiation. 



In this work varietal names have been used only 

 for the most distinct forms, but we have endeavoured 

 to give an idea of the extent of variation exhibited by 

 the British representatives of each species. 



In the synonymy for each species we have endea- 

 voured to give references to all important European 

 books and papers dealing exclusively with the Charo- 

 phyta, to general works in which these plants have 

 had special attention, and to all illustrations of any 

 importance. Specific names bestowed on particular 

 varieties or forms not found in Europe have not been 

 included in the lists of synonyms. 



The citations will, it is hoped, be found sufficiently 

 full to allow of easy reference, in the large libraries, 

 to the books and papers cited. The complete titles 

 will be found in the bibliography included in the 

 second volume. 



The dimensions given of the fruits, antheridia, etc., 

 are the result of measurements of a number of fairly 

 normal specimens. They should be taken as an 

 approximate guide only, since the limits of variation 

 of size in some species are extremely wide, and the 

 figures would lose their significance if extended so far 

 as to include every form. 



