6 



G. O. SAES. 



[No. 8. 



These forms exhibit a great interest, more especially from 

 their perplexing resemblance to certain northern species of Clado- 

 cera. For not only do all of the 5 species belong to well-known 

 northern genera, but most of them are even so closely allied to 

 some of our indigenous forms, that I have found it a matter 

 of no little difficulty to settle with full certainty their specific 

 distinctness. This fact, indeed, is the more surprising, since the 

 region whence they were derived, not only lies thousands of miles 

 distant from our country, located, as it is, in the southern 

 hemisphere, but . is also in its climatological and biological rela- 

 tions totally dissimilar from any in our latitudes, both the fauna 

 and flora exhibiting, as is well known, in that part of theglobe, 

 quite a peculiar character. 



The facts detailed above force upon us the very difficult 

 question, in what manner the distribution of these minute ani- 

 mals can be supposed to have tåken place, and by what means 

 it is still effected. Any possibility of a true active migration, 

 such as might be assumed for the several marine forms, cannot 

 of course be admitted as regards the animals in question; and 

 moreover, as a great part of the Cladocera are confined to small 

 stagnant ponds and ditches, with no direct connexion whatever, 

 even a passive distribution, by the aid of rivers or streams, may 

 to a great extent be precluded. To explain the distribution of 

 such forms, zoologists have assumed, that the ova in the ponds 

 having dried up during the summer season, might possibly in 

 some cases be carried abroad to some distance by the wind along 

 with the finely crumbling mud. Such may, indeed, very often 

 occur; and, considering the tenacious germinative power of the 

 winter-eggs and the rapid reproduction of the individuals during 

 the spring and fore part of the summer, a partial dissemination 

 of the species may very reasonably be assumed to take place in this 

 manner. But, on the other hand, I opine that the very exten- 

 sive range of geographical distribution characterizing certain 

 species of Cladocera, as shown by later investigations. is not 

 sufficiently explained by such an assumption. It has long been 

 known, that at least throughout northern and middle Europe the 



