584 - Heredity and Evolution 



when the days grow steadily longer. Carefully 

 controlled experiments have proved that the 

 stimulus for this ripening of the eggs and 

 sperm often originates from the increased 

 time of exposure to light rather than from 

 an increase in the total amount of light re- 

 ceived. This fact, indeed, has been used by 

 modern hatcheries, which force the brood 

 hens to lay prematurely, using artificial lights 

 to duplicate the approach of spring. Also 

 many plants display a similar photoperio- 

 dicity, putting forth buds, flowers, and fruits 

 on a schedule that is determined mainly by 

 the waxing or waning of the daylight in the 

 different seasons of the year (p. 265). How- 

 ever, other plants are more sensitive to the 

 thermoperiodic fluctuations of their environ- 

 ment. 



Gravity. The gravitational force of the earth 

 has remained quite constant for eras, but the 

 magnitude of this force has influenced the 

 evolution of all organisms. In fact, if the 

 earth had been a significantly larger or 

 smaller planet (such that the force of gravity 

 were correspondingly greater or lesser) the 

 end results of evolution would necessarily 

 have been quite modified. 



Aquatic plants and animals are buoyed up 

 by the surrounding water and thus are pro- 

 tected from the full impact of gravity. Conse- 

 quently the supporting structures of aquatic 

 organisms, compared to terrestrial forms, 

 are not subjected to so great a strain. Some 

 seaweeds, for example, are able to grow four 

 or five hundred feet up from the ocean floor, 

 despite the fact that these algae lack any 

 specialized strengthening tissues such as are 

 present in the higher plants. Among terres- 

 trial plants mainly those that have evolved a 

 capacity to deposit strong annual rings of 

 xylem tissues are able to achieve a very tall 

 treelike form. 



Many aquatic animals possess skeletal ma- 

 terials that are relatively light and weak, 

 such as the cartilage of the elasmobranchs 

 (p. 669) and the delicate porous bones of 

 many fish. But land animals require a heavier 

 skeleton to sustain their fleshy bulk, and this 



imposes a definite restriction upon their 

 growth. In fact, the unwieldy skeletal weight 

 of some dinosaurs is known to have been a 

 handicap that partly accounted for their ex- 

 tinction. 



Pressure. Land organisms do not often ex- 

 perience great changes in the pressure of 

 their surroundings. At sea level the pressure 

 is only about 15 pounds per square inch (one 

 atmosphere), and even at the highest alti- 

 tudes where living things are found, the 

 pressure seldom drops to less than half an 

 atmosphere. But many marine species live at 

 oceanic depths of more than three miles, 

 where the pressure exceeds 8000 pounds per 

 square inch. Here pressure introduces a num- 

 ber of interesting ecological problems. 



Most deep-sea forms, if they are suddenly 

 brought to the surface, cannot survive the 

 change. Fish with swim bladders are particu- 

 larly vulnerable in this respect, although 

 most truly deep-sea forms do not have swim 

 bladders. Nevertheless some fish, living at 

 moderate depths, are literally torn apart, so 

 great is the expansion of the gas in the blad- 

 der when the surrounding pressure is re- 

 duced. But even if the swim bladder is lack- 

 ing, the fish or other deep-sea form is apt to 

 die as a result of decompression; and con- 

 versely, surface forms do not usually survive 

 when exposed to deep-sea pressures. 



An animal may be protected partly from 

 the effects of decompression if it is cooled as 

 the pressure is reduced. Thus deep-sea forms 

 may remain alive and active at sea-level pres- 

 sure if the water in the aquarium is cooled 

 nearly to the freezing point. Low pressure, 

 apparently, is counteracted by low tempera- 

 ture, and such an antagonistic action be- 

 tween pressure and temperature has been 

 demonstrated in many experiments. Gener- 

 ally speaking, the metabolic reactions of an 

 organism are equally sensitive to both pres- 

 sure and temperature, and increasing tem- 

 perature shifts the metabolic equilibria oppo- 

 sitely to increasing pressure. Moreover, the 

 metabolic reactions of deep-sea and surface 

 forms are keyed to different combinations of 



