70 



as we now do that the artic zone houses fresh-water Bryozoa, and 

 that probably other species than those here mentioned may be 

 expected to occur, our researches must aim at a better under- 

 standing af their mode of reproduction. 



It is a well known fact, that by far the greater part of the 

 investigations regarding the reproduction of the lower fresh- 

 water organisms have until a few years ago taken place ex- 

 clusively in Central Europe. Owing to studies pursued in this 

 from a geographical point of view very limited area our know- 

 ledge on this head has been based. It seems to have been 

 tacitly understood that the results arrived at from this territory 

 must hold good in any other locality where the lower fresh- 

 water fauna happened to occur, i. e. almost all over the surface 

 of the earth. During later years a series of publications have 

 appeared on this fresh-water fauna, resting on investigations 

 carried on in artic and North European as well as in the high 

 alpine lakes of Switzerland, in Italy and North Africa. In Central 

 Europe these studies have further been carried on with some 

 regard to the conditions to which the organisms occurring in this 

 territory are subjected. Of this extensive literature we shall here 

 only mention the general results that have been gained, which 

 further researches will more fully substantiate. In all the lower 

 fresh-water groups of animals with various modes of repro- 

 duction — digonic or monogonic (parthenogenesis, gemmation, 

 division) or producing two types of eggs (resting eggs with 

 thick shells and summereggs with thin shells) the two modes 

 of reproduction are not employed indiscriminately under all 

 climatic conditions nor in every locality; nor are the two types 

 of eggs everywhere utilised to the same degree. This is the case 

 with the Copepoda, Cladocera, Ostracoda, Rotifera, Bryozoa, 

 Planaria, Dinoflagellata and will in the future probably likewise 

 prove to hold good with regard to the Phyllopoda. In certain 

 latitudes and under certain conditions the monogenic propagation 

 prevails, in others the digonic. The circumstance that organisms 



