402 BIOLOGY AND ITS MAKERS 
at this time, for she read aloud excellently, and she not only 
took an interest in my theoretical and experimental work, 
but she also gave practical assistance in it.” 
In 1893 he published The Germ-Plasm, A Theory oj 
‘Heredity, a treatise which elicited much discussion. From 
that time 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 plants, especially 
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. 
In his Die Afutationstheorie, published in rg01, 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 variations, and 
points 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 
production of new species” and that ‘as natural selection 
acts solely by accumulating slight, successive, favorable 
variations, 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 
eo thousands of years, but by jumps through sudden, through 
psmall transformations.” (Whitman’s translation.) 
The work of De Vries 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, 
