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LIBftAR Y r 



PREFACE .^^, 



While working in the Lincoln Park Greenhouse 

 in Chicago some forty years ago, the writer made 

 some curious observations concerning the growth of 

 plants. In attempting to find the reason for the failure 

 of plants to grow for a succession of years in the same 

 greenhouse soil, it was found that the addition of vir- 

 gin soil from Wisconsin to the exhausted soil proved 

 of little avail unless the amount of virgin soil mixed 

 with that from the greenhouse was equal to, or greater 

 than, the amount of the latter. Of itself, this consti- 

 tuted a matter of little interest, but in connection with 

 this study the surprising observation was made that the 

 addition of a small quantity of the exhausted soil to 

 the virgin soil resulted in a better growth of the fronds 

 than was obtained in the virgin soil alone. The reason 

 for this remained obscure for a number of years, but 

 the phenomenon excited the writer's interest to further 

 investigations in the field of biology. 



This interest was greatly intensified by a period 

 spent in Europe, where, under the guidance of the 

 great Virchow, the writer's interest was directed 

 towards animal cells and the effect of environment 

 upon them. At that time the majority of the students 

 of biology were greatly influenced by the writings of 

 Darwin and Huxley. In consequence, many investi- 

 gators were enmeshed in the study of "the survival 

 of the fittest," as Herbert Spencer tersely stated the 

 doctrines of Darwin. While recognizing the impor- 



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