146 Chlorophycece 



(1) The structure of the cell-contents is one of the most 

 constant features exhibited by a species; but this fact can be of 

 little classificatory value owing to the very large number of species 

 which possess the same structure and arrangement of the chro- 

 matophores. 



(2) The outward form of the cell, as seen in front view, varies 

 within certain limits, which are usually very small, but which may 

 in exceptional cases (such as in pure cultures) be considerable. 

 The form of the vertical view is, as a rule, a more constant feature 

 than the form of the front view. 



(3) The ornamentation (scrobiculations, granulations, spinu- 

 lations, etc.) of the cell- wall is relatively constant, being always 

 arranged according to a definite law, which is only transgressed by 

 variations in one or more of the individual component groups which 

 constitute the pattern of arrangement. 



(4) The prolific growth and rapid division of immense numbers 

 of Desmids have a tendency to produce variations from the 

 typical forms. 



(5) Changes in the conditions of environment cannot affect 

 the characters of a species unless they act for long periods of time. 



It is most unfortunate that so little is known concerning the 

 geographical distribution of Desmids, as such a knowledge would 

 probably be of much greater value than one would at first sight 

 imagine. I have previously shown that the production of per- 

 manent variation in species of Desmids under natural conditions 

 is much more difficult than is generally supposed 1 ; also, that some 

 of the prettily marked species are found in such widely separated 

 localities as Ceylon, Java, Hong Kong and Queensland, identical 

 in every respect and possessing precisely the same markings 2 . It 

 is likewise most improbable that this result has been brought 

 aboii t by a parallelism of modification in the course of the evolution 

 of these forms, owing to the occurrence of other species with a 

 world-wide distribution and equally constant characters. The 

 distribution of Micrasterias foliacea Bail, is sufficient to settle this 

 point, as the plant in question is the most aberrant of all forms of 

 the genus Micrasterias, having secondarily assumed a filamentous 

 condition ; and yet the plants found in North and South America, 

 India, Burmah, Siam, China, Australia and New Zealand cannot 

 be distinguished from one another. It" should also be borne in 



1 G. S. West, 1. c. p. 371. 



- W. & G. S. West in Trans. Linn. Soc. Bot. ser. 2, vol. vi, 1902, p. 124. 



