74 BEITISH ASSOCIATION FOB 



in space, nor to atmospheric currents simply ; but to some general law connected 

 with the atmosphere of our planet, according to which there is a " self-develop- 

 ment" within it of liviug organisms, which organisms he suspects may have somo 

 relation to the periodical meteorolites or aerolites. The advo<;ates of progressive 

 development may see and hail in this the first step in the series of ascending 

 transmutations. The unbiassed observer will be stimulated by the startling 

 hypothesis of the celebrated Berlin Professor to more frequent and regular exam- 

 inations of atmospheric organisms. Some late examinations of dust showera 

 clearly show them to have a source which Ehreuberg has denied. Some of my 

 hearers may remember the graphic description by Her Majesty's Envoy to Persia, 

 the Hon. C. A. Murray, of the cloud of impalpable red dust which darkened the 

 air of Bagdad, and filled the city with a panic. The specimen he collected was 

 examined by my successor, at the Royal College of Surgeons, Prof. Qutkett, and 

 that experienced microscopist could detect only inorganic particles, such as fine 

 quartz sand, without any trace of Diatomaceje 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, 1857 ; 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. 



To specify or analyze the labours of the individuals who of late years have 

 contributed to advance Zoology by the comprehensive combination of the various 

 kinds of researches now felt to be essential to its right progress, would demand a 

 proportion of the present discourse far beyond its proper and allotted limits. Yet 

 I shall not be deemed invidious if I cite one work as eminently exemplary of the 

 spirit and scope of the investigations needed for the elucidation of any branch of 

 natural history. That work is the monograph of the Chelonian Reptiles (tortoises, 

 terrapenes and turtles) of the United States of America, published last year at 

 Boston, U.S., by Prof. Agassiz. 



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

 ural groups or families of the vegetable kiugdom, and to a clear scientific compre- 

 hension 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 distribution 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 diifusion 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 distance from the source of heat and the obliquity of 

 the sun's rays, modified by altitule above the sea level, or the degree of rare- 

 faction of the atmospiiero 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 influence the distribution of the heat they have absorbed. Thus 

 large tracts of dry land produce dry and extreme climates, while large expanses 

 of sea produce humid and equable climates. Agriculture affects the geographical 



