308 



ANIMALS IN INLAND WATERS 



Bi 



^•Pl. ajpina_ 



^ Pol. cornuta 



OPI. gonocephala 



ft&Sfti* 



^^£^#&>flfc 







IV^-r-^ ^\7S 



■>;v:r;V;;v-..M 



Ci 



j? IG . 84.— Distribution of brook planarians shown on a schematic longitudinal 

 section of a mountain stream. In A, under primitive conditions, Planaria alpina, 

 (I), Polycelis cornuta, (III), and Planaria gonocephala, (V), succeed each other 

 with areas of intermixture (II and IV), between the adjacent species. This 

 original arrangement is altered when the conditions are changed; in B, Polycelis 

 is eliminated by the two Planaria; in C, Planaria alpina is replaced by Polycelis. 

 After Voigt. 



