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, difi'use 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 Algae 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 Algae. 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 Algte 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 shalloAV 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 diifusion 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 

 Nostocliaceas. 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, 



* Otlier substances besides Nostocs occasionally get this name. Masses of undeveloped frog-spawn, 

 for instance, dropped by buzzards and herons, pass for meteoric deposits. 



