Evolution of the Genie Material 483 



thoptera have very large and typically arranged chromosome sets, 

 frequently around 2n = 24 individual ones. Crabs tend to possess very 

 high numbers (over 100) of small, dotlike chromosomes; and Diptera, 

 a few (down to 6) chromosomes arranged in pairs. I cannot think of 

 any reasonable explanation of this, since all types certainly work 

 equally well from the point of view of the mechanism of meiosis and 

 heredity; and since the organizational differences, say between 

 Diptera, Orthoptera, and Lepidoptera, are not of a magnitude that 

 requires different cellular mechanisms. 



Not less enigmatic are the much studied (see White, 1945, 1951 

 for animals; and Babcock and students' extensive work on Crepis, 

 19470,^) relations between karyotypes of species of the same genus 

 or nearly related genera. Sometimes a large number of species have 

 the same or almost the same karyotype (e.g., shorthorn grasshoppers). 

 Even nearly related species sometimes have difiFerent karyotypes (e.g., 

 some grasshoppers), and there are even subspecies with different 

 karyotypes (e.g., domestic and wild silkworms, some Peromyscus) . 

 Much work has been done to find out whether these different karyo- 

 types can be derived from an ancestral one, and this is frequently so. 

 The methods of change seem to be, in most cases, division of the 

 centromere with breaking apart of two arms, union of terminal centro- 

 meres with the opposite effect, all kinds of translocations, and peri- 

 central inversions. We have already mentioned the well-worked-out 

 example of the Drosophila species (see I 3 C d bb), in which many 

 different karyotypes can be derived in this way from a basic con- 

 figuration. Numerous examples for all this can be found in the books 

 by White (1945), Matthey (1949), Patterson and Stone (1952), and 

 Stebbins (1950) for plants, and almost daily in current cytological 

 literature. 



With the exception of polyploid plants, all species and genera of a 

 group may have different karyotypes, and the karyotypes may be used 

 as a taxonomic criterion of value. To a certain extent this is true also 

 for the subgenera of Drosophila. However, such differences are not 

 general and are not sufficiently orderly to give them a definite phylo- 

 genetic meaning in terms of chromosomal evolution. This means that 

 in special cases we might well conclude that, say, a group with six 

 chromosomes is derived from one with eight chromosomes by one or 

 the other known or supposed processes; but no rule obtains which 

 would permit endowing such processes with a genetic meaning, fitted 

 to serve as a basis for evolutionary speculation. (Good examples in 

 Babcock's 1947 Crepis monograph.) It is just this point which makes 



