sential to uiulerstaiuling the relation of an organism 

 to tlie temperature of its environment. 



All organisms outside the tropics must adjust to 

 meet tlie critical winter season. In those species ac- 

 tive throughout the year, there is an increase in re- 

 sistance to cold, hrought ahout, in part, by dehy- 

 dration of body tissues (Payne 1927), or by 

 increase in density of plumage (Kendeigh 1949) or 

 fur (Sealander 1931). Those species incapable of 

 maintaining activity in situ over winter either migrate 

 to more favorable regions or hibernate, or the adults 

 die. Many invertebrates survive the winter in re- 

 sistant egg or larval stages. 



Oxygen 



Dry air at 760 mm Hg pressure contains ap- 

 proximately 21 per cent oxygen, 0.03 per cent carbon 

 dioxide, 78 per cent nitrogen, and traces of other 

 gases. Oxygen is thus much more abundant, con- 

 stant, and available at all times in air than it is in 

 water. Oxygen availability seldom becomes a critical 

 factor for land animals, with the occasional exception 

 of forms that live in the soil or invade high altitudes. 

 Although terrestrial organisms have evolved sim- 

 ple moist chambers, branched tracheal systems, or 

 complicated lungs to replace the gills found in many 

 aquatic forms, the fundamental requirement of moist 

 membranes for the exchange of oxygen and carbon 

 dioxide between body fluids, tissues, and the sur- 

 rounding medium remains the same. The skin still 

 serves this purpose in some terrestrial forms — an- 

 nelids and some amphibians — but in most forms the 

 moist membranes are within the body. Internal place- 

 ment decreases the loss of water through evaporation. 

 The evolution of an ability to take oxygen directly 

 out of the air apparently preceded the actual invasion 

 of land, and may have been induced in the pond and 

 marsh habitat when oxygen dissolved in the water 

 became reduced or absent during summer stagnant 

 periods (Pearse 1950). The evolution of internal air- 

 breathing organs was probably concurrent with the 

 evolution of mechanisms to prevent excessive water 

 loss from the exposed surfaces of the body. 



Solar radiation 



Solar radiation takes the form of an endless 

 procession of waves. The length of a light wave 

 from crest to crest, or trough to trough, determines 

 its character in respect to energy and color ; the 

 height of the wave determines its intensity. Wave- 

 length is commonly expressed in millimicrons (Im^ ^ 

 0.000001 mm = 10 Angstrom units, A). All wave- 

 lengths have a velocity of 299,340 kilometers per 

 second. The solar spectrum varies from 51 m^i, which 



is the shortest ultraviolet radiation, to 5300 m\i, which 

 is the longest infrared radiation. The spectrum vis- 

 ible to man is between 390 m^i and 810 m^. Consid- 

 erable ultraviolet is absorbed by the atmosphere, and 

 the ultraviolet wavelengths re.'iching the earth's sur- 

 face are mostly between 292 mji and 390 m^. Color 

 perception by man is as follows : violet, 390- 422 m^ ; 

 l)lue, 422-492 m(i ; green. 492-535 mn ; yellow, 535- 

 586 mn : orange. 586-647 mn; and red, 647-810 mpi. 

 The longer waves are rich in heat energy ; the shorter, 

 in actinic energy. 



Solar radiation may be measured with a ])yrheli- 

 ometer, by which readings are given in terms of heat 

 energy (g-cal/cm-'/sec). Results are not accurate for 

 the shorter wavelengths. Photoelectric cells accu- 

 rately measure intensities in the shorter wavelengths ; 

 readings are given in foot-candles. The Macbeth il- 

 luminometer measures total sunlight in foot-candles 

 by visual comparison of observed intensity with a 

 standardized and calibrated light source set within 

 the instrument ; accuracy is limited by sensitivity 

 of the human eye. With any photometric instrument, 

 the measurement of intensity of any portion of the 

 spectrum requires the use of calibrated color filters 

 that screen out everything but the desired wave- 

 lengths. 



Different wavelengths have different effects on or- 

 ganisms. Green light is reflected by plants ; little is 

 used in photosynthesis. Some early experiments on 

 tadpoles, fish, snails, and other forms (Davenport 

 1908) indicate that there is an increasing growth rate 

 in different wavelengths, in the following order : 

 green, red, white, yellow, blue, violet. The physio- 

 logical basis of this phenomenon is not known. An 

 excess of infrared may produce overheating of the 

 animal. Ultraviolet in large concentrations is harm- 

 ful to most animals, but in lower intensities is bene- 

 ficial to elaboration of vitamin D. Evidence indicates 

 that ultraviolet combined with rainfall is important 

 in controlling numbers of some mammals, forest-edge 

 birds, and insects. In general, terrestrial organisms 

 are exposed to much higher intensities of solar radia- 

 tion than are aquatic organisms and have evolved 

 horny or chitinous body coverings, hair, or feathers, 

 that function in part to protect internal structures 

 from lethal concentrations. Long-range vision has de- 

 veloped only in land animals and is correlated with 

 the high light intensities characteristic of terrestrial 

 habitats. 



Diurnation 



Animals may be divided into diurnal (day- 

 time), crepuscular (late evening and early morning), 

 nocturnal {nighi) , s.nd arhythmic (irregular) species. 

 .\nimals that occupy microhabitats where tempera- 

 ture and light changes are negligible at most tend to 



Rock, sand, and clay 99 



