746 General and Applied Biology 



lines, depending on one or more differing envii^onniental factors? Is this 

 not illustrative of "descent with change" ? 



In fact, when similar animals or plants having identical inheritance 

 are placed in widely different environments, the development is quite 

 different and characteristic for each. Even human identical twins in 

 whom the inheritance is considered as nearly identical as is possible, 

 when placed in different environments develop into somewhat different 

 types. 



7. Evidences From Heredity and Variations. — The scientific study 

 of heredity and variations has shown through indisputable facts and data 

 that organisms are constantly changing. What is this so-called "descent 

 with change" but an evolving which is fundamental and inherent in all 

 life, regardless of where we may care to place the origin, the responsi- 

 bility, and control of this evolving process? 



If there were no change during descent, what would be the possibility 

 for progress? Is it more remarkable to have created a world, the living 

 contents of which are immutably constant and static, or to have one in 

 which life has the abilities of constantly changing according to natural 

 laws? Since our environmental factors are constantly changing, is it 

 not necessary that our living organisms have the ability constantly to 

 vary in order that they may keep step with the environment in which 

 they are to live, develop, and struggle for their existence? 



B. Theories of Descent With Change 



1. Lamarck's Theory of Acquired Characters (1809). — Lamarck at- 

 tempted to explain differences in individuals by suggesting that through 

 disuse or use for specific purposes certain parts of an organism were 

 under- or overdeveloped and that such differences were later inherited 

 by future offspring. The first part is true — use and disuse modify struc- 

 tures and functions — but even today we have no conclusive evidence 

 that such or similar modifications of the body or somatic cells are in- 

 heritable. 



2. Darwin's Theory of Natural Selection as a Factor in the Origin of 

 Species (1859). — Darwin observed that animals produce larger numbers 

 of offspring than can naturally and normally exist in any given locality 

 but that the total numbers of that particular species remain more or less 

 constant or stationary. 



Because of their morphologic and functional differences (or varia- 

 tions due to descent with change), there ensues between these offspring 



