2 Introduction 



per cent, of the British freshwater Algse could be identified with 

 certainty from Cooke's book ; and Wolle's American publication 

 would be of little or no assistance. The relationships and affinities 

 of the Alga3 described in these publications, and in many cases 

 their life-histories also, are now much better known. This has 

 resulted in great changes in their classification. 



At the time Cooke's book was published it was expected by 

 many that during the next few years the groups of the Protococcacese 

 and ChroococcaceaB would have disappeared, and statements were 

 made by certain authors advocating great polymorphism in Algae, 

 evidence being brought forward to prove that many of the more 

 lowly forms were obviously stages in the development of higher 

 forms. I have elsewhere pointed out 1 that these statements were 

 based upon very inadequate observations and that more evidence 

 is yet required concerning the life-histories of some of the lowly 

 types before any definite statements can be formulated as to their 

 systematic position. Again, because a few observers have at 

 different times suggested and attempted to show that these lowly 

 types are only developmental stages, that constitutes no reason 

 why, when their life-histories are unknown, they should be neglected 

 and left out of consideration. Yet, that is largely the case in Wolle's 

 ' Freshwater Alga? of the United States.' 



Blackman 2 in advocating the primitive nature of the genus 

 Ghlamydomonas, also remarks that " this specific constancy of the 

 most primitive type is in strong opposition to the idea of wide 

 polymorphism brought forward by Hansgirg, Chodat, and Borzi, 

 which associates different genera, and even members of different 

 families, in the life-history of one individual." No one would 

 doubt for a moment the existence of considerable polymorphism 

 in most groups of the Alga?, but it is certainly on a more reason- 

 able scale than was at one time advocated. 



The classification I have adopted is thoroughly explained in 

 the next chapter and the reasons for so arranging these plants are 

 stated in full. 



With regard to the terminology, I have for the most part 

 followed that used by Vines in his ' Text-book of Botany 3 .' 



1 G. S. West, 'Algafl. of Cambridgeshire,' Journ. Bot. 1899, pp. 5253. 



2 F. F. Blackman, 'The Primitive Algas and the Flagellata,' Ann. Bot. xiv, 1900, 

 p. 660. 



3 S. H. Vines, 'A Students' Text-book of Botany,' London, 1895. 



