BURSA BURSA-PASTORIS AND BURSA HEEGERI 

 BIOTYPES AND HYBRIDS. 



Bv GEORGE HARRISON SHULL. 



INTRODUCTION. 



Darwin recognized in the facts of variation a key to the riddle presented 

 by the multiformity and many obvious interrelations of all living" things. 

 Since the appearance of the Origin of Species the observation and discus- 

 sion of variations have assumed a dominant place in biology, and a serious 

 conflict has recently developed regarding the interpretation of the observed 

 facts. It is now generally recognized that this conflict can be brought 

 to a termination only through the application of experimental methods. 

 Inspection alone can not decide the question as to how an observed varia- 

 tion originated and what bearing it may have on the future of the race in 

 which it occurs. Studies in the museum and in the field only discover the 

 fact, and to some extent the range, of variation occurring under a more or 

 less limited and inadequately known range of conditions, and can not cer- 

 tainly determine its cause or causes ; neither can these means supply more 

 than a suggestion based upon insecure inference as to the hereditary nature 

 of any variation. The causes of variation can be determined only by 

 subjecting equivalent material to different controlled conditions, and their 

 hereditary relations can be learned only through the conduct of pedigree- 

 cultures. 



We already know, as a result of experimental work in these directions, 

 that variations are of fundamentally different types, having different causes 

 and obeying different laws of development and heredity. A knowledge of 

 these facts impresses an important principle, namely, that the range of 

 applicability of any conclusion reached by the investigation of one class of 

 material or one characteristic can be determined only by similar experiments 

 with other material and other characteristics. 



This has been one of the guiding principles in the institution of the ex- 

 perimental work at the Station for Experimental Evolution of the Carnegie 

 Institution of Washington. Numerous species of plants and animals of 

 quite wide relationships have been brought under observation, and it may 

 be expected that each will in time give material assistance in determining 

 to what extent principles and hypotheses now available have general validity, 

 and will also lead to the discovery of such new principles, or such modifi- 



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