very few species, at least among poikilo- 

 thermic organisms, that have extended their 

 range of habitat widely enough to include 

 all kinds (tropical, temperate, and frigid) of 

 climatic conditions. 



Lastly, some organisms display a capacity 

 to become dormant during periods of very 

 low or high temperature, and frequently the 

 dormant protoplasm is especially adapted for 

 enduring adverse conditions. This is espe- 

 cially true of seeds, spores, and cysts, which 

 sometimes remain alive after they have been 

 exposed to subfreezing or near-boiling tem- 

 peratures. Usually such a great temperature 

 resistance indicates that the protoplasm has 

 undergone reorganization. The cells elimi- 

 nate part of their normal water content, and 

 in a partially dehydrated condition the pro- 

 teins are less susceptible to denaturation by 

 low and high extremes of temperature. 



Light. Almost all the radiant energy re- 

 ceived by the earth comes from the sun, and 

 if the sun grew dim, all parts of the world 

 would become too cold for the survival of 

 any living thing. Light striking the earth is 

 transformed mainly into heat, and this main- 

 tains the environmental temperature. But a 

 small fraction of sunlight provides energy for 

 the growth of green plants. And since organic 

 compounds synthesized by plants are essen- 

 tial for the sustenance of virtually all other 

 organisms, the importance of light in the 

 general economy of life cannot be overem- 

 phasized. 



A competition for light among green plants 

 is a primary factor in their struggle for ex- 

 istence. Treelike species survive by over- 

 shadowing their smaller competitors, al- 

 though sometimes a smaller species, by dint 

 of numbers, will pre-empt a certain region, 

 making it difficult for the seedlings of a larger 

 species to gain a foothold. Small annual 

 plants may also succeed in growing on the 

 floor of very dense deciduous forests, but 

 such plants tend to sprout very early in the 

 springtime and to produce their seeds before 

 the overlying foliage has had a chance to 

 blanket them from the sun. Thus, one trend 



Ecology and Evolution - 583 



of natural selection among woodland flowers 

 has been toward a very quick and abundant 

 production of seeds. 



Among animals, light has been a primary 

 factor in the development of eyes and other 

 photoreceptive organs, which play a para- 

 mount role in the acquisition of food, the 

 finding of shelter, the avoidance of danger, 

 and the fulfillment of the reproductive func- 

 tions of the species. Creatures that live en- 

 tirely in a dark environment — as in deep 

 caves, underground burrows, or in the abys- 

 mal depths of die ocean — are usually sight- 

 less. This may be due either to a degenera- 

 tion (Fig. 30-10) of photoreceptive organs 

 that once were possessed by the ancestral 

 species, or to a failure of the ancestral spe- 

 cies to develop such organs initially — depend- 

 ing on the earlier evolutionary background. 



The breeding activities of many birds and 

 mammals are initiated in the springtime 



Fig. 30-10. Model of a blind cave-dwelling salaman- 

 der, taken from the Ozark Mountains (see text). (Cour- 

 tesy of the American Museum of Natural History, New 

 York.) 



