402 BIOLOGY AX I) ITS MAKERS 



at this lime, for she read aloud excellenllv, and she not onlv 

 look an inleresl in ni}- iheorelical and experinienlal work, 

 ])ut she also gave ])ractical assistance in it." 



In icSg3 he ]jublished The Gcrui-Plasui, A Theory of 

 Heredity, a treatise which elicited much discussion. From 

 I hat lime on he has been actively engaged in replying to his 

 critics and in perfecting his system of thought. 



The Mutation-Theory of De Vries. — Hugo de Vries 

 (Fig. 115), director of the Botanical Garden in Amsterdam, 

 has experimented widely with the growth of |)lants, especiaily 

 the evening primrose, and has shown that different species 

 appear to rise suddenly. The sudden variations that breed 

 true, and thus give rise to new forms, he calls mutations, and 

 this indicates the source of the name applied to his theory. 



1y\\\\s Die Miitationstheorie, published in igoi, he argues 

 for the recognition of mutations as the universal source of 

 the origin of species. Although he evokes natural selection 

 for the perpetuation and improvement of \ariations, and 

 Doints out that his theory is not antagonistic to that of natural 

 selection, it is nevertheless directly at variance with Darwin's 

 fundamental conception — that slight individual variations 

 "are probably the sole differences which are effective in the 

 productioQ of new species" and that "as natural selection 

 acts solely b\- accumulating slight, successive, favorable 

 \ariations, it can produce no great or sudden modifications." 

 The foundation of De Vries's theory is that "species have 

 not arisen through gradual selection, continued for hundreds 

 or thousands of years, but by jumps through sudden, through 

 small transformations." (Whitman's translation.) 



The work of De \>ies is a most important contribution 

 to the study of the origin of species, and is indicative of the 

 fact that many factors must be taken into consideration when 

 one attempts to analyze the process of organic evolution. 

 One great value of his work is that it is based on experiments, 



