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their biology during the development of genetics that this closely 

 focussed picture of the details of variation has come to be regarded 

 as furnishing an adequate explanation of the whole process of 

 evolution. 



However, for at least half this forty-year period of experi- 

 mental study there have been those who have questioned, not the 

 value or validity of genetics within its field, but its sufficiency as 

 an explanation of the "origin of species," and even more, of its 

 adequacy to account for the appearance and dififerentiation of the 

 higher categories of classification. The significance of "gene muta- 

 tions," the key processes from the geneticist's point of view, is dis- 

 counted; the great majority of those reported are minute in degree 

 and have no survival value, even for minor varieties, let alone for 

 groups of specific rank. 



In this period of challenge and examination of the bearing of 

 "gene mutations" on evolution, Richard Goldschmidt has been a 

 leading figure. A systematist who has studied the gypsy moth in its 

 wide north temperate distribution, and a geneticist who has made 

 contributions of recognized importance to our knowledge of the 

 inheritance and expression of sex, he is convinced that "gene muta- 

 tions" have no bearing on evolutionary changes beyond the grade 

 of races, geographical varieties, and other categories below the grade 

 of species. Such minor differentiation he classes as "microevolution- 

 ary" variation. 



For the production of a new species, a "macroevolutionary" 

 variation in the germ plasm is needed. He finds the genetic basis for 

 such changes in the complete reorganization of chromosomes which 

 he believes is accomplished by translocations, segmental inter- 

 changes, by which he believes not particulate variations are brought 

 about but a whole nexus of changes. The resulting organisms would 

 stand apart as completely dififerentiated new species. In his view, a 

 chromosome is not to be regarded as a grouping of discrete genes, 

 like separate beads on a string, but rather as a unified chemical 

 entity in the nature of a long chain molecule. The complex of char- 

 acteristics of a species arises from the pattern of the chromosomes 

 taken as wholes. Change a few small loci in a chromosome and you 

 have still the old pattern with minor variation ("gene mutations"), 

 but reorganize and redistribute the chromosome parts and you have 

 new "genetic molecules" and the pattern for a different species. 



