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THE AMEBIC AN NATURALIST [Vol.XLIX 



cies and varieties have their chromosome numbers in 

 series 1:2:3:4, etc., it seems by no means improbable that 

 what we have imagined above has actually occurred many 

 times. And if one may believe that the event has the 

 result supposed, all the worry about relationships between 

 chromosome number and height of species in the scale of 

 evolution may be eliminated. 



Normal, Chromosome Behavior and Heredity 

 The second query, concerning the relation of normal 

 chromosome behavior to the transmission of characters, 

 is much more important than the one just examined, but 

 it can be discussed more briefly. By normal " chromo- 

 some behavior" is meant a reduction division where ma- 

 ternal and paternal chromosomes approach each other in 

 definite pairs (if homologous pairs are present), chance 

 only governing the passage of either to a particular 

 daughter cell. This is probably the usual behavior in the 

 higher plants and animals, and upon this behavior depends 

 Mendelian heredity in the narrow sense. The thesis to be 

 submitted and scrutinized is the following: The maximum 

 possible difficult?/ in the improvement of animals and 

 plants by hybridization usually depends directly upon the 

 chromosome number. 



When a mutation in a single determinant takes place in 

 the germ cells of a plant, such as may cause the loss of 

 red color in the corolla, crosses between such a form and 

 the normal give a monohybrid Mendelian result. Two 

 mutations in non-homologous chromosomes gives in a 

 similar way a dihybrid result. Such simple conditions, 

 however, are not met with very frequently. For example, 

 White found that a fasciated tobacco when crossed with 

 the type from which it sprang and from which it probably 

 differed only by this single determinant, gave a mono- 

 hybrid Mendelian ratio in the F 2 generation; but when the 

 fasciated type was crossed with other types the result was 

 a complex F 2 population. This population was suscepti- 

 ble of analysis, nevertheless, and showed that the various 



