18 
INTRODUCTION. 
IV. 
sulphate of copper, and many, perhaps almost all other chemical solutions, 
become filled in time, and under favorable circumstances, with a similar vegetation. 
The waters of mineral springs, both hot and cold, have species peculiar to them. 
Some, like the Red snow plant, diffuse life through the otherwise barren snows of 
high mountain peaks and of the polar regions ; and on the surface of the polar 
ice an unfrozen vegetation of minute Algte finds an appropriate soil. There are 
species thus fitted to endure all observed varieties of temperature. Moisture and 
air are the only essentials to the development of Algte. It has even been sup- 
posed that the minute Diatomacece whose bodies float through the higher regions of 
the atmosphere, and fall as an impalpable dust on the rigging of ships far out at 
sea, have been actually developed in the air ; fed on the moisture semicondensed 
in clouds ; and carried about with these “ lonely” wanderers. 
When this atmospheric dust was first noticed, naturalists conjectured that the 
fragments of minute Algae of which the microscope showed it to be composed, had 
been carried up by ascending currents of air either from the surface of pools, or 
from the dried bottoms of what had been shallow lakes. But a different origin 
has recently been attributed to this precipitate of the atmosphere by Dr. F. Cohn, 
Professor Ehrenberg, and others, who now regard it as evidence of the existence of 
organic life in the air itself ! This opinion is founded on the alleged fact, that 
atmospheric dust, collected in all latitudes, from the equator to the circumpolar 
regions, consists of remains of the same species, and that certain characteristic forms 
are always found in it, and are rarely seen in any other place. Hence it is inferred 
that the dust has a common origin, and its universal diffusion round the earth 
points to the air itself as the proper abode of this singular fauna and flora, — for 
minute animals would seem to accompany and doubtless to feed upon the vegeta- 
ble atoms. If this be correct, and not an erroneous inference from a misunderstood 
phenomenon, it is one of the most extraordinary facts connected with the distribu- 
tion and maintenance of organic life. 
If Algae thus people the finely divided vapour that floats above our heads, we shall 
be prepared to find them in all water condensed on the earth. The species found 
on damp ground are numerous. These are usually of the families Palmellacece and 
JSf ostochacece. To the latter belong the masses of semi-transparent green jelly so 
often seen among fallen leaves on damp garden walks, after continued rains in 
autumn and early winter. These jellies are popularly believed to fall from the 
atmosphere, and by our forefathers were called fallen stars.* If such be their 
origin, we are tempted to address them, with Cornwall in King Lear, 
“ Out vile jelly ! where is thy lustre now?” 
for certainly nothing can well be less star-like than a Nostoc, as it lies on the ground 
An appeal to the microscope reveals beauty indeed in this humble plant, but gives 
no countenance to the popular belief of its meteoric descent. It is closely related 
in structure to other species found under dripping rocks and in lakes and ponds, 
* Other substances besides Nostocs occasionally get this name. Masses of undeveloped frog-spawn, 
for instance, dropped by buzzards and herons, pass for meteoric deposits. 
