Japanese Freshwater Triclails. ß^ 



temperature may be considered as representing a paramuunt cause 

 bringing about tbe condition of the distribution. In Europe the 

 freshwater Triclad-fauna has just been proved as indicating the 

 association with the conditions of the chmate. PL alpina which 

 inhabits cold streams in various locahties, extending fi'om the 

 Alpine regions to Lapland, is generally regarded as the relict of 

 the glacial age. During the height of that period it is t^upposed to 

 have dourished in the plain of Northern Europe, but retreated to 

 higher altitudes as well as to more northern and colder latitudes on 

 the approach of a genial temperature, leaving the plain for a sub- 

 sequent immigrant, Fob/, comuta, which was followed by PL 

 (jonocephala. 



It is an interesting fact that there is a remarkable resemblance 

 in the distribution of the European and Japanese Triclad-faunas. 

 The similarity of physical conditions does not necessai'illy result in 

 a similar fauna, and conversely. To me, however, this parallelism 

 seems to be of particular value in suggesting a view tliat the climate 

 of Japan has been subjected to a certain extent to variations very 

 similar to those ascei'tained on the European Continent. Perhapes 

 PI. vivlda occurred in the low-land of Centrai Japan under con- 

 ditions of climate and temperature similar to those under which 

 PI. alpina flourished in the plain of Nurthern Europe. On the ap- 

 proach of a milder climate PI. vivida retired to higher altitudes as 

 well as to move northern and colder latitudes, giving place to a 

 subsequent immigrant, Poly, aurlcidata. Following the latter, PI. 

 gonocephala made its appearance, extending over almost all the 

 northern hemisphere. In this manner, the parallelism between the 

 European and Japanese Triclad-faunas can be explained. 



In conclusion, I have a few words to state as to Thiexemaxx's 

 theory (77) in respect of the blindness of certain freshwater animals. 

 He has, judging from the facts that some blind Niphargiis and 

 Planar i a- î^l^ecies usually occur in stenothermal water, spawning in 

 winter and revealing their distribution quite parallel to that of PI. 

 alpina, considered them as representing the glacial relicts, and 

 further, he has advanced the view that they are descended from 

 ancestors with eyes, which inhabited drainage water during the 



