THEORY OF EVOLUTION 509 



recognized : ( 1 ) continuous or fluctuating variation, such as height of 

 individuals of a species where they are expected to fall within a nor- 

 mal range thus allowing a degree of variation among individuals of 

 the species, and (2) discontinuous or sport variation (mutation) 

 where the variation falls outside the normal range of variation and 

 not connected with it by intermediate changes. 



In the course of seven generations of this primrose and involving 

 approximately 50,000 individuals, six different mutations were found. 

 The new characters which appeared were quite different from those of 

 the typical species and were hereditary as well. Since this was the case, 

 de Vries concluded that he was observing the origin of new species. 

 He was sufficiently convinced of this to discount Darwin's conception 

 of the development of new species by the gradual accumulation of 

 continuous variations through natural selection. He pointed out 

 that mutations are due to changes occurring in the germ plasm 

 while the continuous variations, individually, are due to changes in 

 somatic cells. 



Following de Vries' work there has been much study of mutations, 

 and numerous ones have been found in nature. Too, it has been 

 found that spontaneous mutations could be produced in Drosophila 

 (fruit fly) by x-ray radiation. It is thought that mutations come 

 as the result of physiological changes in the chromosomes or genes. 



While this mutation theory of origin of new species has prompted 

 much study and thought on evolution it seems not to have displaced 

 Darwin's general conception of the origin of new species. So far as 

 Darwin's theory is concerned, the occurrence of mutations only 

 hastens the process of evolution since they produce quick, abrupt 

 variations instead of the slower, smaller, continuous variations. 

 Natural selection will operate with either. Biologists now consider 

 both small and large variations as mutations, and have turned back 

 to Darwin's idea of natural selection as the most likely explanation 

 of the development of new kinds of animals. 



References 



Lindsey, A. W.: Problems of Evolution, New York, 1931, The Macmillan 



Company. 

 Lull, E. S. : Organic Evolution, New York, 1929, The Macmillan Company. 

 Newman, H. H. : Evolution Yesterday and To-day, Baltimore, 1932, Williams & 



Wilkins Company. 

 Wilder, H. H.: History of the Human Body, New York, 1909, Henry Holt & 



Company, Inc. 



