THE CONCEPT OF EVOLUTION 597 



arguments advanced the theory of uniformitarianism to replace the con- 

 cept of catastrophism. In 1785 James Hutton developed the concept that 

 the geologic forces at work in the past were the same as those operating 

 now. He arrived at this conclusion after a careful study of the erosion of 

 valleys by rivers, and the formation of sedimentary deposits at the 

 mouths of rivers. He demonstrated that the processes of erosion, sedi- 

 mentation, disruption and uplift, carried on over long periods of time, 

 could account for the formation of fossil-bearing rock strata. The pub- 

 lication of John Playfair's Illustrations of the Huttonian Theofy of the 

 Earth in 1802 gave further explanation and examples of the idea of 

 uniformitarianism in geologic processes. Sir Charles Lyell, one of the 

 most influential geologists of his time, finally converted most of the con- 

 temporary geologists to the theory of uniformitarianism by the publica- 

 tion of his Principles of Geology (1832). A necessary corollary of the idea 

 that slowly acting geologic forces have worn away mountains and filled 

 up seas is that geologic time has been immensely long. This idea, com- 

 pletely revolutionary at the time, paved the way for the acceptance of 

 the theory of organic evolution, for the process of evolution requires 

 an extremely long time. 



The earliest theory of organic evolution to be logically developed 

 was that of Jean Baptiste de Lamarck, the great French zoologist whose 

 Philosophie Zoologique was published in 1809. Lamarck, like most biol- 

 ogists of his time, was a vitalist, and believed that all living things are 

 endowed with a vital force that controls the development and function- 

 ing of their parts and enables them to overcome handicaps in the en- 

 vironment. He believed that any trait acquired by an organism during 

 its lifetime was passed on to succeeding generations— that acquired char- 

 acters are inherited. Developing the notion that new organs arise in 

 response to the demands of the environment, he postulated that the size 

 of the organ is proportional to its use or disuse. The changes produced 

 by the use or disuse of an organ are transmitted to the offspring and 

 this process, repeated for many generations, would result in marked 

 alterations of form and function. One of the classic illustrations proposed 

 by Lamarck is the evolution of the long neck of the giraffe. Lamarck 

 suggested that the short-necked ancestor of the giraffe took to browsing 

 on the leaves of trees, instead of on grass, and that, in reaching up, it 

 stretched and elongated its neck. The offspring, inheriting the longer 

 neck, stretched still farther, and the process was repeated untd the 

 present long neck was achieved. 



Both Buffon and Erasmus Darwin had similar ideas about the role 

 in evolution of the direct response of the organism to its environment, 

 but had not expressed them so clearly. This theory, called Lamarckism, 

 provides a fine explanation for the remarkable adaptation of many 

 plants and animals to their environment, but is completely unacceptable 

 because of the overAvhelming genetic evidence that acquned character- 

 istics cannot be inherited. The theoretical distinction between somato- 

 plasm and germ plasm made by Weismann (1887) refuted all theories 

 of evolution based on the inheritance of acquired characters. Acquired 

 characters are present only in the body cells (somatoplasm) and not in 



