THE DARWINIAN THEORY OF EVOLUTION 



373 



of heredity does not show that characters acquired in the life 

 time of an individual organism are inherited. 



Recently, there has been a revival of interest on the part of cer- 

 tain modern scientists in Lamarck's theory. They base their ideas 

 on the supposition that over- or under-development of an organ 

 produces hormones in different amounts, which may affect a gene 

 in the germ plasm and cause variations. 



The Darwinian theory of evolution. The theory of natural 

 selection was set forth in the book, Origin of Species, by Charles 

 Darwin (1809-1882), but it was also formulated independently 

 by Alfred Russel Wallace (1823-1913). Herbert Spencer (1820- 

 1903) has exerted a profound influence upon thinking people 

 through his philosophical writings on evolution. Darwin's theory 

 is based largely upon deductions made from observations that he 



Am. Museum of Natural History 



Restoration is shown of a prehistoric reptile, Allosaurus, one of the largest carnivorous ani- 

 mals that ever lived. It was forty-seven feet long with its head twenty feet above the ground 

 when it stood upright. Below is the skeleton shown as unearthed. From its position, scientists 

 think that it was feeding on one of the giant herbivorous dinosaurs just before its death. 



