260 THE PHILOSOPHY OF BIOLOGY 



are the only parts of the earth and its envelopes which 

 they do not inhabit. 



The chlorophyllian organisms include those uni- 

 cellular plants and animals — the distinction becomes 

 obscure with regard to these organisms — which are 

 pigmented blue, green, brown, or red owing to the 

 existence in the cells of chlorophyll, or of some substance 

 allied to this compound, and they include, of course, 

 the green plants. Like the Bacteria their distribution 

 is world-wide, extending over land and sea and fresh- 

 water areas ; and it is restricted mainly by the distribu- 

 tion of sunlight, and by a lower limit of temperature. 

 The Marine Algae, the Diatoms, the Peridinians, and 

 other chlorophyll-containing organisms appear to in- 

 habit all parts of the world ocean, certainly within a 

 depth of about twenty to fifty fathoms from the surface 

 of the sea. Green plants inhabit the land everywhere 

 except within polar areas, the tops of high mountains, 

 and over areas desert by reason of lack of water, or 

 by the presence of mineral substances. 



These conditions — temperature, light, soil, etc. — do 

 not appear to limit the distribution of the Arthropods 

 and Vertebrates. We find both kinds of animals in 

 the deepest oceanic abysses (deep-sea fishes and Crus- 

 tacea), in polar land and sea regions (Man, some 

 Insects, Crustacea, and Birds), as well as in desert areas 

 and on the summits of the loftiest mountains. The 

 Ants share the subsoil with the Bacteria. Birds and 

 Insects conquer the atmosphere by their activity and 

 not, like the Bacteria, merely by being blown about. 

 Crustaceans such as the Copepoda have much the same 

 distribution in the sea as the Insects have in the atmo- 

 sphere, while Isopods and Amphipods are a parallel, 

 so far as the sea bottom is concerned, to the Spiders, 

 Millipedes, and Ants on the land. Fishes are distributed 



