106 Dynamic Theory. 



enormous changes and preparations. The growth of vegetation in gen- 

 eral ceases, and plants reduce themselves to the smallest compass by 

 shedding their leaves, or even their stems and every part above ground. 

 Some animals prepare themselves by constructing warm retreats, into 

 which they convey stores of food, or in which they hibernate in a torpid 

 and inactive state. All change their clothing whether it be natural or 

 artificial, and vast multitudes migrate to more congenial latitudes. All 

 this is caused by the fact that the axis of the earth is not perpendicular 

 to the plane of its orbit. The plants and animals of the polar regions 

 are very different from those of the equatorial regions, those of moun- 

 tains and high plateaus differ from those of low plains and marshes, 

 they of dry and barren countries are not in general like them of moist 

 and productive regions, the inhabitants of the sea are not like those of 

 the land. There is in all these cases obvious adaptation of the organism 

 to its surroundings ; the fish to the water, the quadruped to the land, 

 the amphibian to both, the bird to the air, &c. These adaptations are 

 not all equally perfect. If we could imagine that they ever were, it is 

 certain they could not remain so for the reason that the surroundings or 

 environment of the organisms is forever subject to change. Geological 

 history is full of proofs of this, as we have seen. The same spot has 

 been successively a deep saltwater sea, a dry plain, a fresh water marsh, 

 a lake and a high mountain. 



Its temperature in these several states may have varied from the heat 

 of boiling water to arctic coldness ; and from being azoic at one time it 

 may have afforded at other times conditions for the support of shell fish 

 and sea weeds, or tropical jungles and hideous reptiles, or herbiverous 

 mammals and prowling carnivores. While these changes occur, and we 

 are to remember they are occuring to-day as much as ever they were, 

 two things happen ; one is the migration of a part of the population and 

 the other is the modification of the rest. The migrations of plants and 

 many lower animal forms is effected slowly and gradually by the birth 

 of the new generations on that margin of the habitat which remains 

 most favorable for its life, and the cessation of such births on the un- 

 favorable margin. Plants whose seeds are carried by winds or currents 

 of water may thus move many rods or even miles in a generation. 

 Shell fish with small locomotion may move but a few yards in a genera- 

 tion. Vertebrate animals are equipped for more rapid migration. 



It scarcely need be argued that animals or plants remaining in a 

 changing environment will be modified by it, provided the change is not 

 too rapid. If it is too rapid they will die. 



Different species of the same genus are to be found in both very hot 

 and very cold countries ; all of which are nearly related by blood, and 

 yet an exchange of habitat between them if made suddenly would prove 



