A CENTUEY OF ZOOLOGY IN AMEEICA 437 



part of the body has been affected by one or another of 

 these mutations." To arrange these mutations arbi- 

 trarily into graded series would give the impression of an 

 evolutionary series, but this is directly contrary to the 

 known facts concerning their origin, for each mutation 

 " originated independently from the wild type." " Evo- 

 lution has taken place by the incorporation into the race 

 of those mutations that are beneficial to the life and 

 reproduction of the individual." This evolutionary 

 process is usually accompanied by the elimination of 

 those forms which have remained stable or which have 

 developed adverse mutations. 



A question that is being vigorously debated at this 

 time concerns the possible effects of selection on the 

 hereditary factors. Are the genes fixed both qualita- 

 tively and quantitatively or does a given gene vary in 

 potency under different conditions and in different indi- 

 viduals I In the former case selection can only separate 

 the existing genes into separate pure strains. But if the 

 gene be quantitatively variable, then selection will result 

 in the establishment of new types. 



Castle has long stoutly maintained the effect of such 

 selection, and his forces have recently been augmented by 

 Jennings. The experimental work now in process will 

 doubtless yield a decisive answer. 



Conclusion. 



A comparison of the simple descriptive natural history 

 of a century ago with the foregoing manifold develop- 

 ments of modern biology will indicate the wonderful 

 progress which has occurred during this period. The 

 path has led from the crude methods of the almost 

 unaided eye and hand to the applications of the most 

 delicate experimental apparatus. For the marvelous 

 success which zoology has attained has been possible only 

 by the skillful use of scalpel, microscope, microtome and 

 other mechanical devices and |)y the refined methods of 

 the chemist and physicist. 



The central truth to which all these discoveries consist- 

 ently point is the unity and harmony of all biological 

 phenomena, and indeed of all nature. No longer does the 

 zoologist find any demarcated line separating his field of 



