ZOOLOGICAL ZONES 1521 



mented, far-seeing, and genial philosopher, Edward 

 Forbes, has provisionally defined twenty-five. 



The same physical conditions are associated with 

 a certain similarity between the animals of different 

 provinces. Where those provinces are proximate, 

 such likeness is due to the identity or close affinity 

 of the species; but where the provinces are remote 

 the resemblance is one of analogy, and species of 

 different genera or families represent each other. 



A second mode of expressing the ascertained facts 

 of the geographical distribution of marine animals 

 is by tracts called Homoiozoic Belts, bounded by 

 climatal lines; which are not, however, parallel with 

 lines of latitude, but undulate in subordination to 

 climatal influences of warm or cold oceanic currents, 

 relations of land to water, etc. Of these belts Pro- 

 fessor E. Forbes has defined nine: one equatorial, 

 with four to the north and four to the south, which 

 are mutually representative. 



But the most interesting form of expression of the 

 distribution of marine life is that which parallels 

 the perpendicular distribution of plants. Edward 

 Forbes, availing himself of the valuable results of 

 a systematic use of the dredge, first showed that ma- 

 rine animals and plants varied according to the 

 depth at which they lived in a manner very anal- 

 ogous to the changes in the forms and species of 

 vegetation observed in the ascent of a tropical moun- 

 tain. He has expressed these facts by defining five 

 bathymetrical zones, or belts of depth, which he 

 calls: i. Littoral; 2. Circumlittoral; 3. Median; 

 4. Infra-median; 5. Abyssal. 



