GRAVITY, PRESSURE, AND SOUND 



129 



SUMMARY 



Light is a complex environmental factor 

 that produces diverse ecological effects. 

 First of all, it supplies energy for the photo- 

 synthesis of carbohydrates by green plants. 

 Although this food synthesis is fundamental 

 for the existence of animals, the whole 

 process is largely taken for granted in the 

 present account. Many organisms, plants as 

 well as animals, react definitely to photic 

 stimulation. Vision is important in the life 

 of the majority of higher animals, whether 

 insects or vertebrates. The direct effects of 

 light on growth, development, and survival 

 are mentioned, but not discussed at length. 



Animals as well as plants may be affected 

 by seasonal (or experimental) changes in 

 the length of the daily period of illumina- 

 tion. Examples of both show long-day and 

 short-day effects that express themselves in 

 animals, by gonadal activation or regression 

 among many seasonal breeding forms, by 

 migration as in birds, and by pelage or 

 plumage changes, including marked altera- 

 tion in color. In certain aphids, the appear- 

 ance of winged forms and the change from 

 parthenogenetic to sexual reproduction may 

 depend, among other influences, on the rel- 

 ative length of day (p. 123), Although 

 other environmental factors, such as tem- 

 perature, may control seasonal changes 

 in plants or animals, photoperiodicity is fre- 



quently of prime importance. The precise 

 significance of its influence can be deter- 

 mined only by direct tests for the given 

 kind of organism. As might be expected, 

 the eyes are the chief receptors concerned 

 in the photoperiodicity of animals. 



In a much different field, there is the 

 matter of the penetration of light into aquat ■ 

 ic habitats. Extinction occurs in sfight 

 depths in fresh waters, along sandy or 

 muddy marine coasts, and, for a different 

 set of reasons, in polar parts of the oceans 

 exposed to the more slanting rays from the 

 sun. 



So far as vision is concerned, sunhght is 

 replaced to a sUght degree by biolumines- 

 cence in the otherwise aphotic depths of the 

 sea. The animal inhabitants of different 

 depths show fairly regular differences in 

 color. The eyes are larger in the zone of 

 perpetual daytime twifight, and are smaller 

 or even lacking in forms from still deeper 

 water. 



Finally there is the matter of ultraviolet 

 radiation from the sun, which never reaches 

 the earth's surface at wavelengths shorter 

 than 2900 A; the atmospheric cut-off is 

 usually effective at somewhat longer wave- 

 lengths. Lethal effects of ultraviolet rays on 

 viruses, bacteria, fungi, and eggs of animal 

 parasites are important for many animals, as 

 are also the vitamin syntheses produced 

 under exposure to such radiation. 



8. GRAVITY, PRESSURE, AND SOUND 



The mechanical forces in the environment 

 of animals consist primarily of gravity and 

 pressure, currents of air and of water, and 

 sudden jars that provide mechanical shock. 

 These constitute the molar forces. Sound is 

 a closely related environmental factor, but 

 will be discussed separately. Mechanical 

 forces may impinge on the animal with 

 great uniformity, as does gravity, or they 

 may vary dramatically, as winds that range 

 from dead calm to hurricane force. Run- 

 ning water also ranges from a gentle, steady 

 flow that is almost imperceptible to the full 

 turbulence of waters in flood. Waves vary 

 enormously in pounding power. The less 

 spectacular of these forces are not neces- 

 sarily the less important. 



GRAVITY 



The direction of pull of gravity is invar- 

 iable, and the intensity is also constant foj 

 any given location. The force of gravit) 

 varies with the distance from the center of 

 the earth. This variation is not great enough 

 to effect even those animals that by swim- 

 ming or ffight, by mountain climbing, or by 

 being carried in currents, travel from equa- 

 tor to pole or move vertically from sea level 

 to the height of Mt. Everest. For a given 

 animal, change in relation to other environ 

 mental factors as a result of such vertical 

 translocations dwarfs the effects produced 

 by a variation in the pull of gravity. 



The constancy of gravity makes analytical 



