404 General Biology 



ever, a vast difference of opinion as to the limitations within which 

 evolution operates, both in the individual and in the race, as well as to 

 the method by which evolutionary changes come to be what they are. 



Secondly, if all present-day forms have sprung from ancestors unlike 

 themselves, the question arises as to whether all the different phyla 

 sprang from one original living being (whether evolution is mono- 

 phyletic or monogenetic), or whether there were numerous "first forms" 

 from which all successive forms spring (polyphyletic or polygenetic 

 evolution). 



Having settled for ourselves whether evolution is a fact, we set 

 about trying to find a theory which will account for that fact. 



Only two great theories have been advanced. One by Charles 

 Darwin, known as Darwinism, or Natural Selection, and the other by 

 Korschinsky, and De Vries, known as the Saltation or Mutation theory, 

 or heterogenesis. 



Darwinism holds that, as we have seen, the offspring of a single pair 

 of flies will be almost six billion in ninety days if all eggs were to hatch. 

 It follows that were such increase to continue in all animals and plants, 

 the food-supply would soon become exhausted. There must, therefore, 

 be a struggle for existence to determine which plants and animals are 

 fittest to survive. 



Everyone has noted that millions of eggs, maggots, and insects 

 never reach the adult form of life because they are eaten by various 

 animals. The number of flies and other insects is, therefore, dependent 

 upon the number and activity of their natural enemies as well as their 

 own physical ability to avoid such enemies, together with their ability 

 to obtain a sufficient supply of food and water for themselves. 



It follows that there will be a struggle even among the same group 

 of organisms for food and water, while the whole group must struggle 

 against their many natural enemies. Nature, through such struggle, 

 selects the strongest and most active (as these are the only ones which 

 will not succumb to the struggle) to carry on the race. The particular 

 characteristics which make it possible for plants and animals to survive 

 in this struggle for existence, are said to have a survival value. 



Darwin accepted variations in all living organisms as a fact, and 

 built his theory on that fact. He contended that useful variations by 

 possessing a survival value were transmitted to the offspring of such 

 organisms, so that each succeeding generation received the advantage of 

 its parents' acquired characteristics. 



However, it is generally held now that acquired characteristics are 

 practically never inherited, and that natural selection only explains why 

 certain organisms did not die and others did. It cannot explain the 

 origin of new species. 



Darwinism is based on the assumption that very minute changes 

 are constantly taking place in the organism. These changes have an 



