EVOLUTION 315 



Geographic changes such as are known to have occurred in the 

 past have been potent factors in modifying the struggle for 

 existence and therefore the characters of the survivors. When 

 islands are separated from continents, the problem of living 

 becomes changed for those left on the island and may result in 

 the development or preservation of unique characters. Thus 

 the peculiar fauna of Australia is attributed to the fact that 

 Australia was cut off from Asia at the beginning of Cenozoic 

 times. In this case isolation fostered the preservation of egg- 

 laying mammals comparable to those alive in times immediately 

 preceeding the Cenozoic era. Such Australian mammals pre- 

 sumably failed to survive in other parts of the world because of 

 inability to meet competition of more favorably endowed forms. 

 Changes of consequence to the inhabitants occur when a con- 

 tinent is cut in two, or when two continents or smaller bodies of 

 land become joined by an isthmus. The formation of mountain 

 ranges by eruptions in the earth's surface results in barriers as 

 effective as water for some animals and may bring in its wake 

 changes in climate. When new conditions of living arise, 

 animals must meet them or perish. Granted that the proper 

 variations materialize at the proper time to enable living things 

 to overcome new obstacles to existence, the fittest of each 

 generation will in the course of time come to differ so much from 

 the original stock as to be distinct species. Evolution results 

 from the survival of the fittest which in turn results from the 

 accumulation of favorable variations in each generation. 



Variation. — Darwin's stand on the question of the inheritance 

 of acquired characters was uncertain, but the acceptance of this 

 principle is not vital to the theory of natural selection. Natural 

 selection is the "preservation of favorable individual differences 

 and variations, and the destruction of those which are injurious." 

 Darwin offered no explanation for variability — variability is 

 axiomatic. The minimum amount of variation between two 

 human individuals is found in identical twins, which are twins 

 that have developed from a single egg. Identical twins are of 

 the same sex and show a striking correspondence in physical 

 and mental traits. Fraternal twins, each of which develop from 

 a single egg, may be as unlike as any brothers or sisters born at 

 different times. Variation among individuals of the same species, 

 developing from different eggs, is a fact many times verified. 



