No. 612] GENESIS OF ORGANIZATION OF INSECT EGG 711 



6. Mendelian Factors and Cytoplasmic Organization 

 The central biological problem of the present time is 

 the method of evolution, and a knowledge of the mech- 

 anism of heredity has long been recognized as necessary 

 for its solution. The results derived from breeding ex- 

 periments with the fruit fly, Drosophila ampelophila, have 

 dominated the field of genetics for the past five years, 

 but although of very great interest and importance, their 

 evolutionary significance is not yet certain. To be of 

 primary value from this viewpoint it is necessary to 

 prove that new species may arise by means of Mendelian 

 characters (mutations) such as white eye, miniature wing, 

 club wing, etc. Since no one has ever been able to define 

 satisfactorily what a species really is and hence what 

 characters should be considered of specific value, this is a 

 difficult problem. 



The definitions given by two of our foremost authori- 

 ties, one a systematist and the other a geneticist, are as 

 follows : The systematist writes 



in form, color and arrangement of parts under natural conditions, which 

 are recognizable from descriptions and figures, should receive distinctive 

 names and be catalogued, provided, of course, that the assemblage of 

 characters includes all ontogenetic changes. If, in the examination of 

 abundant material from different natural environments, we find these 

 characters fairly constant, the forms may properly be called species; 



The geneticist writes 



Species may thus be distinguished by peculiarities of form, of num- 

 ber, of geometrical arrangement, of chemical constitution and prop- 

 erties, of sexual differentiation, of development and of many other prop- 

 be found distinguished from other species. 



The mutations that have appeared in Drosophila do 

 not become recognizable until a late stage in the life his- 

 tory of the individual, and are about the last characters 



35 Williston, 1908, Amer. Nat., Vol. 42. 

 sflBateson, 1913, "Problems of Genetics." 



