38 KAARF MÜNSTKK STRØM. M.-.\. Kl. 



V. CONCLUSIONS AND SUMMARY. 



Af'tci' the plankton of Central ICiii-o])c had been examined chiefly by 

 Gernian and Swiss investigators, at the beginning of the present century, it 

 was generally believed that by far the greater part of the plankton orga- 

 nisms were wholly ubiquitous. 



This premature conclusion, based as it was upon, geographically speaking, 

 extremely limited observations, was unfortunate!}' supported by prominent 

 expert authorities and thus became a scientihc dog>iia. At the time when 

 Messrs. William and George Stephen West's epoch-making treatises on 

 the Scottish (later on also on the IiTsh and English) phytoplankton appeared, 

 several specialists were therefore disposed to reject their theories concerning 

 the dependency of the plankton on geological formations and the rain fall. 



Well-known authors were of the opinion that all the characteristic 

 Scottish species of the freshwater plankton described (this concerns mainl}- 

 Desuiidiaccce) were merely tycholimnetic forms originating in bogs and peat}' 

 moors surrounding the lakes or washed off' by the rain from favourable 

 habitats in the drainage area. 



This is without doubt correct as far as a great many Desmids is con- 

 cerned but the existence of a large number of genuine plankton forms is 

 no less a definite fact. 



When other parts of the World were subsequently investigated (especi- 

 ally lakes in Africa and Australia) it became evident that there was a richDesmid- 

 plankton with many eulimnetic forms. — They were not, however, the same 

 as those in the British plankton but belonged to quite different geographical 

 communities of the Desmidiaceæ. 



Before we proceed farther we must therefore deal with the peculiari- 

 ties in connection with the geographical distribution of the Desmids. 



The Desmidiaceæ are among the very few freshwater Algæ that present 

 a really geographical distribution. — A great many Desmids (perhaps as 

 much as one-third of them) are absolutely ubiquitous and of these some 

 species occur both in the swamps of the Tropics and in the glacier lakes of 

 the Polar countries, but the greater part of them are limited to certain 

 regions covering greater or smaller parts of the World's surface. 



The geographical distribution of other freshwater Algæ is as a whole 

 not checked by distance. There are, of course, among them Algæ which 

 will only grow in calcareous water, and others which can only thrive well 

 in soft water ; some others again are able to stand dry weather at certain 

 times of the year but others want it permanently dampish ; some only get 

 along well at high temperatures and consequently grow in the Tropics, 



