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 Sponffice, siliceous Polycystineoe, 
and calcareous Foraminifcra. 
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 PolycystimcR and Foraminifera. 
The soundings which have been brouglxt up from various parts of the Atlantic and 
the Grulf of Mexico, consist chiefly of similar microscopic polythalanious shells, 
mingled with a greensand composed of casts of Foraminifcra. Thus the mode in 
which 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 ppccies. Exotic species are not distinguishable from the 
British ; difference of climate seems not to afl'ect or relate to specific difference, 
and the same exemption from such influence through the minute size and ample 
structure of the Diatomaceas, seems to have been the chief condition of their 
geological longevity as species. 
The progress of our knowledge in the geographical distribution and relations of 
plants and of animals are reviewed at great length. 
The sum of these relations rests on the assumption thateach species h.as been 
created, or come into being, but once in time and space ; and that its present 
diffusion is the result of its own law of reproduction, under the diffusive or 
restrictive influences of external circumstances. These circumstances 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 radiatu 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 Id^-'i Fahr. to one 
of 36°'5 for the month of July. 
Another mode is the classification 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 (iiflferent countries have frequently ditt'erent, 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 proportion of plant-species 
peculiar to them. When 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 eartli has been divided into twenty-five 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 approxi- 
mating to Australian, Soutli African, and Antai'Ctic forms ; and that of Australia, 
characterised by its Eucalypti and Fpacrides, chie&y 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 sea than in its vegetable form. 
