Mr Dana on the Geographical Distribution of Zoophytes, 41 



tell liini to apply liis knowledge of the mode of extracting tin ore from 

 his own gravel to the drift and debris on the flanks of the great north 

 and south chains of Australia, or any smaller parallel ridges of that 

 great country ;* for great would be my pleasure to learn, that through 

 the application of Cornish skill, such a region should be converted 

 into a Ijritish *'E1 Dorado." 



llequesting you to pardon this little digression, which, after all, 

 may be turned to profit, and hoping that you will bo as proud as I 

 am of the connection which is now established between Cornwall and 

 Siluria, believe me to be, my dear Sir Charles, yours most faith- 

 fully, 



Rod. I. MuRCHisoN. 



Geographical Distribution of Zoophytes. By JAMES D. Dana. 



Heat, light, pressure, and means of subsistence, influence 

 more or less the distribution of all animals ; and to these 

 causes should be added, for water species, the nature or con- 

 dition of the water, wlietiier fresh or marine, pure or impure, 

 still or agitated. Next to the character of the water, heat 

 is the most prominent limiting agent for marine animals, 

 especially as regards latitudinal extent ; while light and hy- 

 draulic pressure have much influence in determining their 

 limits in depth. 



Although these causes fix bounds to species and families, 

 they do not necessarily confine tribes of species to as small 

 limits. This is sometimes the case, and it is nearly true of 

 a large group of zoophytes ; yet other tribes and orders in- 

 clude species whose united range comprises all the zones, 

 from the equator to the polar ices, and every depth, to the 

 lowest which man has explored, affording traces of life. 



Order Jlydroidea, — The Hydroidea are met with in all seas, 

 and at great depths, as well as at the surface. The tropics, 

 and the cold water of the frigid zone, have their peculiar 

 species, and a few are found in fresh waters. The rocks and 

 common marine plants of the sea-coast, the dead or living 

 shell, or the floating fucus of the ocean, are often covered 

 with these feathery corals ; and about reefs they occasionally 



* It appears that gold oro has been also found in the metalliferous ridges 

 of South-western Australia (Adelaide, &c.). 



