204 



Mr. W. West and Dr. G. S. West. 



[Oct. 30, 



recent formations. Hence the dearth of Desmids in the lakes. On the other 

 hand, the Scandinavian lakes are situated on the old formations, and contain 

 an abundance of Desmids, many of which are identical with those of the 

 British lake-areas. 



The Desmids of the 'plankton have without doubt originated from the Desmid- 

 community of the surrounding area, although in most cases there is little 

 resemblance between the plankton-community and that which can be observed 

 in the surrounding drainage-basin. There is an almost complete absence from 

 the surrounding peat-bogs and dripping rocks of those species which are most 

 conspicuous and abundant in the plankton. The common Desmids of the 

 bogs are only found in the limnetic region of the lakes as casual or adven- 

 titious constituents, and therefore the great majority of the Desmids brought 

 by the rains into this limnetic region, with its new conditions of life, find it 

 impossible to maintain their further existence, and rapidly perish. The 

 plankton-community as a whole, as shown by Wesenberg-Lund, is a very 

 ancient one, and this is further confirmed in the case of the British lakes by 

 the existence of this distinct community of plankton-Desmids. We have 

 already stated* that many of the plankton-Desmids " have existed under 

 these pelagic conditions for a long time, as there is every indication of this 

 in the modifications some of them have undergone, and in the species and 

 varieties which are at present only known to occur in the plankton." 



During the vast period in which Desmids have been washed by the rains 

 from their bog habitats into the lakes, a specific selection has taken place, 

 certain species having adapted themselves, with or without slight morpho- 

 logical changes, to a limnetic existence. Only those have survived which 

 were able to withstand the new conditions. 



One of the principal conditions necessary for existence in the new life 

 would be the ability to float in the surface waters. In some species, more 

 especially in the discoid species of Micrasterias, this necessity has brought 

 about no morphological alteration ; and in others, amongst which are certain 

 species of Cosmarium and Staurastrum, there is again no change of external 

 form, but a copious development of surrounding mucilage. In many others 

 morphological changes have occurred, mostly in the further development of 

 those characters which have proved of most avail in the struggle against 

 sinking. It is thus that spines and processes have been greatly increased in 

 length, so that many of the plankton -forms are the longest-spined forms 

 known.f Certain species of Staurastrum and Arthrodesmus best exhibit this 



* W. and G. S. West, in 1 Eoy. Soc. Edin. Trans.,' vol. 41, 1905, p. 512. 

 t In the ordinary habitats of Desmids, and in the former habitats of plankton-species 

 spines are commonly found well developed. This armature has probably two functions, 



