changing environment, in case they do not migrate or perish. Protective coverings must be 

 taken on, or discarded, changes in the structure, size and form of the various organs must 

 occur in order to adjust them to their new conditions. If these changed conditions are favor^ 

 able an increase in size and numbers will result, but if unfavorable the size will be reduced 

 and the number of individuals diminished. A barrier \\\ nature is any physical feature that 

 tends to keep the forms of one region separate from those of another, as a mountain range, 

 desert, body of water, etc. When such a barrier is broken down by any of the above men- 

 tioned agencies forms of animals and plants are allowed to mingle, the biological equilibrium 

 is disturbed and an entire readjustment is demanded. Unwittingly man has helped plants 

 and animals over these natural barriers to his sorrow. Illustrations are furnished by the 

 English sparrow, the gypsy moth, the San Jose' scale and the Russian thistle in America; the 

 mongoose in Jamaica and the rabbits in Australia. 



2. WATER. In the ocean and other large bodies of water changes in thx* depth may 

 occur through oscillations of the bottom or variations in the level of the surface. Wind, run- 

 ing water, ice, animals and plants often conspire to lessen the depth of lakes and seas. 

 Climatic changes upon land, the shifting of winds and currents may materially change the 

 temperature of the water in any particular region. The water may become more or less 

 charged with sediment, salt or some other ingredient of sea or lake water. The average 

 amount of oxygen or of carbon-dioxide may be the subject of change, the amount of light may 

 vary and the character of the sea bottom may be slowly or rapidly changed. In all cases plants 

 and animals become adapted to certain average conditions and as these change the animals and 

 plants, that do not perish or migrate, will also change in their efforts to adjust themselves to 

 their new environment. It seems probable that the fresh water plants and animals were 

 derived from marine forms that were isolated in separate bodies of water, the character of which 

 was gradually changed by streams. Barriers also occur in the water consisting of depths and 

 shoals, cold or warm currents, streams of fresh or muddy water, bodies of land, etc. The 

 breaking down of these barriers may occur through the operation of agencies already discussed, 

 leading to the same results as upon land. The opening of the Panama Canal will partially 

 destroy the barrier which has kept the Atlantic and Pacific forms apart and allow a com- 

 mingling of forms with results difficult to predict. 



D. Natural Selection. Many of the changes referred to above as occuring amongst 

 plants and animals take place through the operation of four principles, known as the "principles 

 of natural selection." Other principles are also operative but these are so fundamental and 

 important that they should be understood by all. Through their operation the plants and 

 animals have been many times changed during the long earth history, those that survive being 

 the merest "handful to the tribes that slumber in its bosom." 



1. PRINCIPLE ONE. No two individual plants or animals are exactly alike. Where 

 they seem to be alike a very little observation will show that they are different in every respect. 

 For every difference that man may detect with his special senses we may be certain that many 

 others exist which he is unable to recognize. We may go still further and say that no two 

 parts of the same individual are exactly alike ; no two leaves, blades of grass, flowers or seeds. 

 Try to disprove this statement by finding t\vo things that really are alike. 



2. PRINCIPLE TWO. There is a constant struggle in progress amongst plants and ani- 

 mals. This arises from their imperfect adjustment to their environment and from the fact 

 that more individuals are produced than can be accommodated upon the earth. Plants are 

 struggling for room, food, moisture, and sunlight, against plant and animal enemies, often 

 against unfavorable climate and soil and to secure cross-pollination and distribution for its 

 seed. Animals are struggling for food and water, against plant and animal enemies, to pro- 

 tect their young and often against unfavorable climatic conditions. Only under exceptional 

 circumstances do they struggle for room and air. 



i3. PRINCIPLE THREE. In general, those forms which are best fitted for the particular 

 environment that they are called upon to occupy are the ones that survive. If the struggle is 

 sufficiently severe all others are exterminated. Those characteristics that "best fit" any or- 

 ganism for this struggle will depend upon the structure, habits and environment of such 

 organism. For the plants rapidity of germination will often be an advantage ; a type of root 

 development that will best anchor the plant and enable it to most successfully wrest from the 

 soil its mineral foods and moisture ; a growth above ground which best enables the plant 



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