564 EVOLUTION 



lost. Since the environment is continually changing, animals 

 are forced to change their habits and this results in a change in 

 the use of organs and consequently in the modifications of 

 organs. Lamarck assumed that the modifications in organs due 

 to changes in the habits of animals are transmitted- to future 

 generations and thus perpetuated. The differences between race 

 horses and heavy cart horses, he attributed to the difference in 

 the work enforced upon the ancestors of these two types of horses. 

 As a result of being forced to develop speed, horses used for racing 

 gradually became so modified in form as to be adapted to running, 

 while those forced to draw heavy loads became heavily built 

 and adapted to heavy draft purposes. The modifications in 

 each generation were slight but they were transmitted to the 

 next generation where they became more pronounced. Thus 

 through a series of generations in which the individuals became 

 a little more modified and thus a little better adapted to the work 

 they had to do, the two types of horses gradually became distinctly 

 different. He thought long legged water birds had developed 

 from short legged ancestors, as a result of the effort to stretch 

 while wading. Wading birds, tempted to the deeper water in 

 quest of food, stretch their legs to keep their feathers dry and 

 according to Lamarck's theory this effort to stretch has resulted 

 after many generations in the development of long legs. The 

 webbed feet of ducks and geese developed in response to the 

 effort to keep afloat by spreading the toes apart. By spreading 

 the toes the membranes between them were stretched and gradu- 

 ally became expanded to form the web. In reaching for leaves 

 on high branches, the neck of the giraffe was gradually length- 

 ened. Snakes gradually lost their limbs and their bodies be- 

 came long and slender as a result of their effort to lie close to the 

 ground and creep through small spaces. Thus in being compelled 

 to adjust themselves to a changing environment, Lamarck ex- 

 plained the modification of animals in various directions, and in 

 this way he accounted for the origin of the various species of ani- 

 mals. In plants, which are more passive than animals, Lamarck 

 attributed the modifications to the direct influence of environ- 

 ment. The peculiar shape which some flowers have he attributed 

 to the pressure of the insects visiting them. Differences in tem- 

 perature, light, food supply, etc., directly modify plants. Lamarck 

 thought that the modifications are transmitted from one genera- 



