XIV 



EXPERIMENTAL STUDY OF THE INFLUENCE 

 OF ENVIRONMENT ON ANIMALS 



BY JACQUES LOEB, M.D. 



Professor of Physiology in the University of California. 



I. INTRODUCTORY REMARKS. 



WHAT the biologist calls the natural environment of an animal is 

 from a physical point of view a rather rigid combination of definite 

 forces. It is obvious that by a purposeful and systematic variation 

 of these and by the application of other forces in the laboratory, re- 

 sults must be obtainable which do not appear in the natural environ- 

 ment. This is the reasoning underlying the modern development 

 of the study of the effects of environment upon animal life. It was 

 perhaps not the least important of Darwin's services to science that 

 the boldness of his conceptions gave to the experimental biologist 

 courage to enter upon the attempt of controlling at will the life- 

 phenomena of animals, and of bringing about effects which cannot 

 be expected in Nature. 



The systematic physico-chemical analysis of the effect of outside 

 forces upon the form and reactions of animals is also our only means 

 of unravelling the mechanism of heredity beyond the scope of the 

 Mendelian law. The manner in which a germ-cell can force upon 

 the adult certain characters will not be understood until we succeed 

 in varying and controlling hereditary characteristics ; and this can 

 only be accomplished on the basis of a systematic study of the effects 

 of chemical and physical forces upon living matter. 



Owing to limitation of space this sketch is necessarily very in- 

 complete, and it must not be inferred that studies which are not 

 mentioned here were considered to be of minor importance. All the 

 writer could hope to do was to bring together a few instances of the 

 experimental analysis of the effect of environment, which indicate the 

 nature and extent of our control over life-phenomena and which also 

 have some relation to the work of Darwin. In the selection of these 

 instances preference is given to those problems which are not too 

 technical for the general reader. 



