Ecologic Study of Living Organisms 627 



trlbution upon which animals may be dependent. Very strong and con- 

 stant winds may cause certain animals to change their habitats and seek 

 a more quiet environment. Wind afTects the ecologic relationships of 

 plants in such ways as pollination (transfer of pollen from male part of 

 the plant to the female part), in helping to disperse the seeds of many 

 varieties, and in supplying movements of plants (can it be called exer- 

 cise?) which may be necessary for the development of such plants. 

 Winds might also aid in the distribution of oxygen, carbon dioxide, and 

 obnoxious gases which in various ways might influence plant activities. 



(d) Gravity: Unless land animals have special mechanisms by means 

 of which they can counteract gravity, the heavier ones will be distributed 

 at or near the surface. If animals move away from the surface, they 

 do so in opposition to the force of gravity. Hence, the heavier animals 

 are usually found at a lower level than the lighter ones. In water cer- 

 tain animals seem to be very little affected by gravity, for they are found 

 at various depths. Other animals seem to live only near the surface, 

 while others live on or near the bottom. Undoubtedly, other factors, 

 besides gravity may influence these distributions of animals in various 

 depths of the water. The force of gravity affects all plants either posi- 

 tively or negatively, sending stems and leaves upward and roots down- 

 ward. Gravity in all probability affects aquatic organisms and some- 

 what determines their vertical distribution, depending on the specific 

 gravity or density of the organisms in question. 



(e) Alternate Recurrence of Day and Night: It is well known that 

 the distribution of animals is quite different in daylight than at night. 

 For instance, certain insects are to be observed only in the daytime 

 (diurnal), while others are found more abundantly at night (nocturnal). 



There are many other factors besides the presence or absence of light 

 which influence animal distributions as just described. The temperature 

 is usually lower at night. This, in addition to more moisture at night, 

 may cause certain animals to be seen at such times. This is particularly 

 true for those types which have no special equipment to prevent the 

 rapid evaporation of moisture from their surface. The stimulating effect 

 of sunlight also may influence the distribution of certain types of animals. 



(f) Physical Make-Up of the Soil: Some soils do not possess specific 

 foods which are very essential to the growth of certain plants, because 

 these foods are not in a form which these plants can utilize. Some kinds 

 of soils are too hard for certain organisms and others are too loose or 

 yielding for others. Certain animals require certain types of soil for 



