(7) Contact.— Animah and plants respond to the 



stimulus of contact. The effect may be ob- 

 served directly upon protoplasm itself, and m- 

 directly in movement. 



(8) Gravity.— In plants the influence of gravity is very 



evident in determining the direction of growth 

 of the chief members. Among the lower ani- 

 mals too the geotropic influence is often quite 

 marked. 



(9) Use and Disuse.— Th^t organs develop with use 



and deteriorate with disuse are facts well 

 known, and variations of considerable size un- 

 doubtedly arise by this process. The differ- 

 ence between the expert and the common man 

 is largely due to the use or disuse of organs and 

 faculties. 



Chapter 4— DISTRIBUTION OF ANIMALS AND 



PLANTS 



(a) — De Candolle's Law 



This law relates to adaptation to climate. Through 

 the action of Natural Selection, plants have become gradu- 

 ally adapted to the climate in which they live, and ill adapt- 

 ed to climates north or south. Just how far modifications 

 may be due to heredity is not clear. After making a care- 

 ful study of this matter De Candolle "concluded that native 

 forms are not hardy when taken one hundred miles north or 

 south of their source." 



(b) — Laws of Distribution of Animals 



"The laws governing the distribution of animals are 

 reducible to three very simple propositions. Every species 

 of animal is found in every part of the earth having condi- 

 tions suitable for its maintenance, unless: — 



(a) Its individuals have been unable to reach this 

 region, through barriers of some sort; or 



(b) Having reached it, the species is unable to main- 

 tain itself, through lack of capacity for adaptation, 

 through severity of competition with other forms, or 

 through destructive conditions of environment; or 



(c) Having entered and maintained itself, it has be- 

 come so altered in the process of adaptation (or as a 

 result of other processes) as to become a species 

 distinct from the original type." 



32 



