404 EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 



account the number of stations included in each zone of depth we find 

 that the number of species per station decreases gradually from 02.8 

 species per station in the shallowest zone to 9.4 species per station in 

 the deepest zone, as shown in the following table: 



Depth in fathoms. 



Species per 

 station. 



Under 100 



62.8 

 51.2 

 30.9 

 24.0 

 15.6 

 10.6 

 9.4 



100 to 500 



500 to 1,000 



1,000 to 1,500 



1, 500 to 2, 000 



2,000 to 2,500 



Over 2,500 





Again, it is interesting to point out that the proportion of genera to 

 species procured in the different zones increases gradually with increase 

 of depth, the ratio of genera to species in the shallowest zone being as 

 1 to 2.93 and in the deepest zone as 1 to 1.17, as shown in the following 

 table : 



Depth in fathoms. 



Ratio of 

 genera to 

 species as — 



Under 100 



1 to 2. 93 

 1 to 2. 37 

 1 to 1. 67 

 ltol.50 

 1 to 1. 45 

 1 to 1.36 

 1 to 1. 17 



100 to 500 



500 to 1,000 



1,000 to 1,500 



1,500 to 2,000 



2,000 to 2,500 



Over 2, 500 





An analysis of dredgings at similar depths close to and far removed 

 from continental shores shows that both species and individuals are 

 more numerous on the terrigenous deposits close to the shore, and the 

 proportion of species to genera is higher, than on the pelagic deposits 

 far removed from the land. This seems to indicate that migration has 

 taken place from the shallow waters close to the shore to the deeper 

 waters of the great ocean basins, and that the ancestors of the fauna 

 at great depths far removed from land have migrated from many 

 shallow-water areas on the surface of the globe. On the whole the 

 deep-sea fauna resembles that of the shallow waters of the polar 

 regions much more than that of the shallow waters of the tropical 

 regions, in so far as the animals of the deep-sea fauna have a relatively 

 small quantity of carbonate of lime in their shells and skeletons, the 

 proportion of genera to species is higher than in the tropics, and there 

 is an absence of x><4agic or free swimming larvae 



In depths of over 1,000 fathoms the Challenger 's trawlings rarely 

 yielded over 10 or 15 specimens of any one species, but in lesser depths, 



