PREFACE. 
The study of the marine Algæ engaged my interest at an early period. 
Originally certain morphological, cytological and physiological questions were the 
objects of my studies, but later the plan to procure a general view of the Algæ 
found in the Danish waters gradually developed. In 1890 I began to make syste- 
matic collections in the Danish waters and continued during the following years, 
especially in 1891—95, when I became able to make extensive dredgings in all the 
Danish waters inside Skagen (Kattegat to Baltic) by means of official support during 
4 years (from “Kommunitetet”) and permission from the ministry to sail with the 
fishery control-steamers S.S. “Havernen” and S. S. “Falken” and the fishery in- 
spection-ship, the gunboat “Hauch”. During the following years I have as occa- 
sion offered continued these collections partly onboard the Biological Station’s S. S. 
“Sallingsund”, especially on a cruise round Bornholm in 1901, the life-saving 
steamer S.S. “Vesterhavet” and the lightship transport S.S. “Nordsgen” in 1905 in 
the North Sea, the deep-sea research-ship S. S. “Thor” in the Skagerak, Kattegat 
and the Sound in 1907, a former revenue-cutter “Ragna” in private possession in 
1904—1906 and partly in fishing boats, especially at different places on the north- 
western coast of Jutland. 
The reason why my work has extended over such a long period is chiefly, 
that different works regarding Greenland’s flora, vegetation and marine Algæ, have 
during a series of years taken up so much of my time that until 1898 I was 
mostly obliged to content myself with collecting material, while the working up of 
this could not begin until after that time. Another cause of the slow progress of 
the work is the abundance of the material collected, and lastly the scope of my 
work was gradually somewhat enlarged. From the beginning the aim of my 
investigations was, not only to state what species are found in the Danish waters, 
but to elucidate their extension here and also their variation and if possible their 
dependence on the external conditions. While working with the single species my 
investigations came more and more to have to do with morphology and the de- 
velopmental history, and I saw how desirable it would be for the task I had 
undertaken if I could contribute as much as possible to the elucidation of the 
natural history of the separate species on the whole in Danish waters; I have also 
expressed this in the title of my work. I feel quite well, that I have not given 
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