THE ADVANCEMENT OE SCIENCE. 75 



distribution of plants, both directly and indirectly. It diffuses plants over a -wider 

 area of eqnal climate, augments their productiveness, and enlarges the limits of 

 their capaf-lty to support different climatal conditions. Agriculture also effects 

 local modifloations of climate. Certain species of plants require more special 

 physical conditions for health ; others more general conditions ; and their extent 

 of diffusion varies accordingly. Thus the plants of temperate climates are more 

 widely dilTused over the surface of the globe, because they are suited to elevated 

 tracts in tropical latitudes. There is, however, another law which relates to the 

 original appearance, or creation of plants, and which has produced different species 

 flourishing under similar physical conditions, in different regions of the globe. 

 Thus the plants of the mountains of South America are of distinct species, and 

 for the most part of distinct genera, from those of Asia. The plants of the tem- 

 perate latitudes of North America are of distinct species, and some of distinct 

 genera, from those of Europe. The Cacteoa of the hot regions of Mexico are re- 

 presented by the Euphorbiacecs in parts of Africa having a similar climate. The 

 surface of the earth has been divided into twenty-live regions, of which I may 

 cite as examples that of New Zealand, in which Ferns predominate, together with 

 generic forms, half of which are European, and the rest approximating to Austra- 

 lian, youth African, and Antarctic forms; and that of Australia, characterized by 

 its Eucalypti and Epaerides, chiefly known to us by the researches of the great 

 botanist, Robert Brown, the founder of the Geography of Plants. 



Organic Life, in its animal form, is much more developed, and more variously 

 in the soa, than in its vegetable form. Observations of marine animals and their 

 localities have led to attempts at generalizing the results ; and the modes of 

 enunciating these generalizations or laws of geographical distribution are very 

 analogous to those which have been applied to the vegetable kingdom, which is 

 as diversely developed on land as in the animal kingdom in the sea. The most 

 interesting form of expression of the distribution of marine life is that which 

 parellols th3 p<irpendicular distribution of plants. Edward Forbes has expressed 

 this by defining five bathymetrical zones, or belts of depth, which he calls,— 

 1, Littoral ; 2, Circumlittoral ; 3, Median ; 4, Infra-median ; 5, Abyssal. The 

 life-forms of those zones vary, of course, according to the nature of the sea-bottom; 

 and are modified by those primitive or creative laws that have caused representa- 

 tive species in distant localities under like physical conditions, — species related by 

 analogy. Very mtich remains to be observed and studied by naturalists in differ- 

 ent parts of the globe, under the guidance of the generalizations thus sketched 

 out, to thn completion of a perfect theory. But in the progress to this, the results 

 cannot fail to be practically most valuable. A shell or a sea weed, whose relations 

 to depth are thus understood, may afford important information or warning to the 

 navigator. To the geologist the distribution of marine life according to the zones 

 of depth, has given the clue to the determination of the depth of the seas in which 

 certain formations have been deposited. Had all the terrestrial animals that now 

 exist diverged from one common centre within the limited period of a few thou- 

 sand years, it might have been expected that the remoteness of their actual locali- 

 ties from such ideal centre would bear a certain ratio with their respective powers 

 of locomotion. With regard to the class of Birds, one might have expected to 



