30 BIOLOGICAL SOCIETY OF WASHINGTON. 



Stood by the assumption that at some remote epoch or epochs, -(but 

 not probably at any one time,) there was a distribution of land 

 which eventually permitted an emigration and immigration of types 

 from one and into another. The absence or slight intrusion of such 

 forms into the northern continent suggests the doubt as to a perfect 

 continuity of the land or fresh-water systems. There are, however, 

 doubtless other explanations available. 



Thus far our attention has been confined to the inland — terres- 

 trial and aquatic — animals, and these alone have been relegated to 

 definite realms or regions by Messrs. Wallace and Allen. The 

 marine faunas of the globe have been practically ignored by both of 

 these students, their attention having been chiefly devoted to the 

 inland regions. The marine realms, however, are entirely inde- 

 pendent of the fluvio-terrestrial, and their characteristic associations 

 of animals are determined by other factors. The groupings which 

 I shall now adopt were first outlined by me in 1875,* ^^^^ certain 'of 

 the fundamental facts were long ago appreciated. 



The importance of attention to the temperature of the sea, and 

 especially to that during the coldest month of the year in the two 

 hemispheres, in relation to zoogeography, was, perhaps, most fully 

 appreciated at first by the illustrious and versatile Dana, who, 

 just about thirty years ago, published, for the time, an excellent 

 '' Isocrymal Chart to Illustrate the Geographical Distribution of 

 Marine Animals" and an accompanying memoir, j in which zoo- 

 geographical principles were discussed with eminent learning and 



wisdom. The chart is still one of the most useful in existence for 

 the student of zoogeography, but later ones answering the purpose, 



and embodying more recent observations, have been published by 



* Theodore Gill. On the Geographical Distribution of Fishes. <^Ann. 

 and ^L1g. Nat. Hist., (4,) vol. 15, p. 251-255; April, 1875. See also John- 

 son's New Universal Cyclopaedia, vol. 2 and 4, and The Nation, vol. 24, pp. 

 27-29, 42, 43; July 12 and 19, 1S77. 



f James D. Dana On an Isothermal Qceanic Chart, illustrating the geo- 

 graphical distribution of marine animals. <^Am. Journ. So. and Arts, (2,) vol. 

 16, pp. 153-167, 314-327; .Sept. and Nov., 1853. 



