30 ANNUAL OF SCIENTIFIC DISCOVERY. 



of Diatomacese or other organic matter. Dr. Lawson has obtained a similar 

 result from the examination of the material of a shower of moist dust or 

 mud which fell at Corfu, in March, 18^7 ; it consisted for the most part of 

 minute angular particles of a quartzose sand. Here, therefore, is a field of 

 observation for the microscopist, which has doubtless most interesting 

 results as the reward of persevering research. 



Observations of the characters of plants have led to the recognition of the 

 natural groups or families of the vegetable kingdom, and to a clear scientific 

 comprehension of that great kingdom of nature. This phase of botanical 

 science gives the power of further and more profitable generalizations, such 

 as those teaching the relations between the particular plants and particular 

 localities. The sum of these relations, forming the geographical distributions 

 of plants, rests, perhaps at present necessarily, on an assumption, viz., that 

 each species has 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 influence of external circumstances. These 

 circumstances are chiefly temperature and moisture, dependent on the dis- 

 tance from the source of heat and the obliquity of the sun's rays, modified 

 Ly altitude above the sea-level, or the degree of rarefaction of the atmosphere 

 and of the power of the surface to wastefully radiate heat. Both latitude 

 and altitude are further modified by currents of air and ocean, which influ- 

 ence the distribution of the heat they have absorbed. Thus large tracts of 

 dry land produce dry and extreme climates, while large expanses of sea pro- 

 duce humid and equable climates. Agriculture affects the geographical dis- 

 tribution of plants, both directly and indirectly. It diffuses plants over a 

 wider area of equal climate, augments their productiveness, and enlarges the 

 limits of their capacity to support different climatal conditions. Agriculture 

 also effects local modifications 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 diffused 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 temperate latitudes of North America are 

 of distinct species, and some of distinct genera, from those of Europe. The 

 Cactere of the hot regions of Mexico are represented by the Euphorbiacere in 

 parts of Africa having a similar climate. The surface of the earth 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 approximating to Australian, South African, 

 and Antarctic forms ; and that of Australia, characterized by its Eucalypti 

 and Epacrides, 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 sea, 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 arc very analogous to those which have been applied to the veg- 

 etable kingdom, which is as diversely developed on land as in the animal 



