BRITISH ASSOCIATION MEETING. 



495 



In micro-geology recent progress has been made : — 



The microscopic organisms hitherto observed in the oldest fossiliferous deposits, 

 Silurian greensands, for instance, are spicula of Spongioa, siliceous Polycystinece, 

 and calcareous Foraminifera. 



Ehrenberg has discovered that the substance of the greensands in stratified 

 deposits, from the Silurian to the Tertiary periods inclusive, is composed of the 

 casts of the interior of the microscopic shells of Pohjcystinece. and Foraminifera. 

 The soundings ivhich have been brought up fi'om various parts of the Atlantic and 

 the Gulf of Mexico, consist chiefly of similar microscopic polythalamous shells, 

 mingled with a greensand composed of casts of Foraminifera. Thus the mode in 

 vrhich a deposit was made at the bottom of the great ocean of the Silurian period, is 

 illustrated by that which the microscope has demonstrated to take place under 

 similar conditions at the present day. 



Perhaps the earliest indubitable evidence of diatoms has been obtained from the 

 Eocene strata; and the forms here determined have been, for the most part, 

 identified with existing species. Exotic species are not distinguishable from the 

 British ; difference of climate seems not to aflect or relate to specific difference, 

 and the same exemption from such influence through the minute size and ample 

 structure of the Diatomacece, seems to have been the chief condition of their 

 geological longevity as species. 



The progress of oui- knowledge in the geographical distribution and relaiions of 

 plants and of animals are reviewed at great length. 



The sum of these relations rests on the assumption thateach species has been 

 created, or come into being, but once in time and space ; and that its present 

 diffusion is the result of its OAvn law of reproduction, under the diffusive or 

 restrictive influences of external circumstances. These cii'cumstances are chiefly 

 temperature and moisture, dependent on the distance from the source of heat and 

 the obliquity of the sun's rays, modified by altitude above the sea-level, or the 

 degree of rarefaction of the atmosphere, and of the power of the surface to waste- 

 fully radiate heat. Both latitude and altitude are further modified by currents of 

 air or ocean, which influence the distribution of the heat they have absorbed. 



The modes of generalizing the observations on the geographical distribution of 

 indigenous plants are various. 



One is by dividing the horizontal range of vegetation into zones, bounded by 

 annual isothermal lines, as 1, equatorial; 2, tropical ; 3, subtropical; 4, warmer 

 temperate ; 5, cooler temperate ; 6, subarctic ; 7, arctic ; 8, polar, zones : with 

 temperatures progressively falling from an annual isotherm of TO^-S Fahr. to one 

 of 36° -5 for the month of July. 



Another mode is the classitication of plants according to the regions of altitude ; 

 as into those of, — 1, Palms ; 2, Tree-ferns : 3, Myrtles ; 4, Ever-greens ; 5, Deci- 

 duous trees ; 6, Conifers ; 7, Alpine shrubs ; 8, Alpine herbs. But the corres- 

 ponding altitudes in different countries have frequently different, though analogous 

 or representative species. The presence or otherwise of snow on the mountain- 

 tops also influences the character of the plants at corresponding altitudes. 



A thiid, and perhaps more truly natural, mode of expressing the geographical 

 distribution of plants, is by regions defined by the prox'Ortion of plant-species 

 peculiar to them. ^\'hen one half, at least, of the known species are peculiar to a 

 certain space, it constitutes a " phytogeographic" region, according to Schouw. 

 In it, also, a fourth part of the genera must be either peculiar, or so predominating 

 as to be comparatively rare in other regions ; and the individual families of 

 plants must be either peculiar tO; or decidedly predominate in such region. 



So defined, the surface of the earth has been divided into twenty-five regions, 

 of which I may cite as examples that of Xew Zealand, in which Ferns predominate, 

 together with generic forms, half of which are European, and the rest approxi- 

 mating to Australian, South African, and Antarctic forms ; and that of Australia, 

 characterised by its Eucalypti and ^acric/es, chiefly known to us by the researches 

 of the great botanist, Robert Bi'own, the founder of the " Geograpb}^ of Plants." 



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

 in the sea than in its vegetable form. 



