398 Coe — A Century of Zoology 'in America, 



Darwinism. ' ' In his later years he gave the theory his complete, 

 though reluctant, acceptance. 



A modified Lamarckian doctrine was widely accepted 

 in the last quarter of the century, due largely to the 

 influence of Cope, Hyatt and Packard. The inheritance 

 of "acquired characters" demanded by this theory seems 

 incompatible with the discoveries of recent times, so that 

 "today the theory has few followers amongst trained 

 investigators, but it still has a popular vogue that is wide- 

 spread and vociferous." 12 



The origin of new varieties and species by accidental 

 modifications of the germplasm is now the most widely 

 accepted theory of evolution. 



Some of the most important discoveries regarding the 

 origin of new forms have been recently made by Morgan 

 and his pupils. From a stock of the common fruit fly 

 (Drosophila ampelophila) more than 125 new types have 

 arisen within six years. Each of these types breeds true. 

 "Each has arisen independently and suddenly. Every 

 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 ? 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 bv 

 Jennings. The experimental work now in process will 

 doubtless yield a decisive answer. 



12 Morgan, T. H. A critique of the theory of evolution, p. 32, 1916. 



