THE ADAPTATION OF ANIMALS TO THE ENVIRONMENT 787 



are needed to insure the pollination of the plant or the dispersal ol its 

 seed. 



356. Adaptations of Species to Species 



The evolutionary adaptation of each species has not occurred in a 

 biologic vacuum, independently of other organisms. On the contrary, 

 the adaptation of each species has been influenced markedly by the 

 concurrent adaptations of other species. As a result of this, many types of 

 interdependencies between species have arisen, some of the clearest and 

 best understood of which involve insects. Insects are necessary for the 

 pollination of a great many plants; the plants are so dependent on these 

 insects that they are unable to become established in a given region 

 unless those particular insects are present. The Smyrna fig, for example, 

 could not be grown in California, even though all climatic conditions 

 were favorable, until the fig insect, which pollinates the plant, was 

 introduced. Birds, bats, and even snails serve as pollinators for some 

 plants but insects are the prime animals with this function. Flowering 

 plants have evolved bright colors and strong fragrances, presumably to 

 attract insects and birds and ensure pollination. There has been some 

 doubt as to whether insects can detect these colors and odors, but the 

 experiments of Karl von Frisch (p. 338) show that honeybees, at least, 

 can differentiate colors, shapes and scents and are guided in their visits 

 to flowers by these stimuli. 



Some of the species to species adaptations are so exact that neither 

 one can exist without the other. The yucca plant and the yucca moth 

 have evolved to a state of complete interdependence. The yucca moth, 

 by a series of instinctive acts, goes to a yucca flower, collects some pollen 

 and takes it to a second flower. There it pushes its ovipositor through 

 the wall of the ovary of the flower and lays an egg. It then carefully 

 places some pollen on the stigma. The yucca plant is fertilized and 

 produces seeds on a few of which the larva of the yucca moth feeds. 

 The plant produces a large number of seeds and can easily spare the 

 ones eaten by the moth larva. 



357. The Distribution of Animals 



Three major habitats can be distinguished, marine, fresh-water and 

 terrestrial. No animal is found in all three major habitats and, mdeed, 

 no animal is found everywhere within any one of these. Every species 

 of animal and plant tends to produce more offspring than can survive 

 within the normal range of the organism. There is a strong population 

 pressure tending to force the individuals to spread out and become es- 

 tablished in new territories. Competing species, predators, lack of food, 

 adverse climate and the unsuitability of the adjacent regions, perhaps 

 due to lack of some requisite physical or chemical factor act to counter- 

 balance the population pressure and prevent the spread of the species. 

 Since all of these factors are subject to change, the range of a species 

 may change suddenly. The range of a species tends to be dynamic rather 

 than stati? The spread of a species is prevented by geographic barriers 



