The Expansion of Evolutionary Concepts 3 



Hiitton, Lyell, and others were making great strides toward an 

 understanding of geology. Lyell, especially, pointed out the gradual 

 nature of the changes involved in geologic events. Contempora- 

 neously, the English clergyman Malthus wrote a pioneering book 

 on human sociology entitled Essay on Population, which em- 

 phasized the checks put on human population by war, disease, and 

 famine. 



In spite of the attacks on Lamarck's theories, even in the early 

 nineteenth century, the idea of the origin of new species through 

 change gained many supporters. Certain authors went so far as to 

 postulate natural selection of favored variations as the mechanism 

 involved in the individual examples or groups which had come to 

 their attention. Climactic to these efforts, the naturalists Charles 

 Darwin and Alfred Russell Wallace arrived independently at the 

 conclusion that natural selection was a general theory explaining 

 the evolution of all life. By prearrangement, their papers were read 

 in London at the same meeting of the Royal Society in 1858. 

 Darwin's classical exposition of this theory. The Origin of Species 

 by Means of Natural Selection, was published in 1859. Although 

 bitterly contested, this book established the Darwin-Wallace theory 

 of natural selection and with it the theory of the evolution of life. 



That some sort of hereditary mechanism existed was realized by 

 Darwin and his colleagues, but its nature was unknown. Mendel, 

 in his now famous study of peas, formulated the basic laws of 

 inheritance in 1865 and made the first great stride in a knowledge 

 of heredity. His work lay neglected and virtually unknown until 

 1900 when these same laws were rediscovered independently by 

 Correns in Germany, De Vries in Holland, and Tschermak in 

 Austria. A little later, Sutton and Boveri independently pointed out 

 the probability that the chromosomes of the cell afforded a mecha- 

 nism for the observed facts of hereditary characters. Soon after, 

 commencing in 1910, T. H. Morgan demonstrated the fact that 

 genetic determinants for many characters occur in a definite linear 

 arrangement along the chromosomes. In the meantime, De Vries 

 and others had outlined the mutation theory. Thus was born the 

 field of modern genetics. 



During the first decades of exploration in the new field of genetics, 

 considerable controversy arose as to whether the laboratory mutants 

 of the geneticists were the minute differences postulated by evolu- 

 tionists as the kind of changes responsible for evolution. Although 

 the controversy still is not entirely settled in the minds of some 

 investigators, by the middle of the twentieth century, a practical 



