GROWTH. ETC., IN THE DESMIDIACEA). 279 



General degeneration of type, then, resulting from rapid and 

 exhausting, because oftentimes almost continuous, division is the 

 great law of Desmid life, and it may naturally be asked — To what 

 extent does this hold good, how far can it affect the size, form and 

 ornamentation of the cell? It has been far too much taken for 

 granted that it did so only to a slight extent. Each characteristic 

 and well-known form is itself so variable within certain narrow 

 limits that it has come to be understood, entirely erroneously, how- 

 ever, that these minute variations, with a zygospore to match, con- 

 stituted the sum of its life history, outside of which it did not move. 

 The discovery, too, that many Desmids have a major and minor 

 form (sometimes also a forma maxwia and forma minima) of ex- 

 actly the same shape, has perhaps strengthened the idea that 

 identity in outline is the sign of identity in species, the minor form 

 being supposed to pass into the major by a process, not of develop- 

 ment through varying types, but by one of ptire growth equally in 

 all directions — a thing that can hardly be said to have any place 

 at all in Desmid life. 



In other words, we have considered the Desmids in some ways 

 too much in the light of the gi"owth of the higher orders of plants, 

 in which the production of seed is the end and aim of existence and 

 their life history a continuous upward growth to a sexual maturity ; 

 whereas among the Desmidiaceae the succession of polymorphic 

 forms, which constitutes the life history of any species, exhibits 

 in the characteristics of the several forms a gradual degeneration 

 from a perfect sporangial type, and the objective of desmid life is 

 not the production of zygospores but the multiplication of cells. 

 Everything is arranged to give way to this, just as everything gives 

 way to the production of seed in the flowering plant The fact 

 •does not seem to have received sufficient recognition that among 

 the microscopic forms of life, both animal and vegetable, with 

 which the waters teem, polymorphism of the most extensive kind 

 plays a leading part. In no family, however, does it more com- 

 pletely' dominate the whole life history than among the Desmids. 

 A true Desmid species consists of an immense number of distinct 

 polymorphic forms, which are partly successive modifications of 

 the sporangial type under stress of repeated cell-division, partly 

 abnormal (but in no sense monstrous) forms brought about by 

 unusual combinations of circumstances, and partly types arising 

 from all these as the result of their struggle to develop upwards 

 towards the perfect exemplar of the species. 



Size. — The question, then, as to how far degeneration affects 

 the size, form and ornamentation of the Desmids may be answered 

 in one word, viz., that under conditions favourable to its operations 

 it never ceases to affect them till it has reduced all three to the 

 simplest possible. It would be interesting to know just how far 

 down the cells of a species could go without being entirely deleted. 

 There seems to be some natural limit, as the smallest forms that I 

 know of are all just about the same dimensions. They are Cylin- 

 drocvsfis ^so-called) minutissima, Turn., 9 x lu, the smallest varia- 



