136 THE BIOLOGY OF INSECTS 



very nearly complete half-and-half example." According 

 to Poulton's interpretation these effects can be produced 

 because the formative pupal tissues retain to some extent 

 the embryonic potentialities : *' the pupal factors which 

 determine the secondary characters of sex are in a condition 

 analogous to that of a Mendelian heterozygote . . . and 

 the underlying characters are revealed by a correctly timed 

 mechanical shock." The shock is apparently able to affect 

 even in the developing pupa these factors, which are 

 continuous, we may believe, with the factors of the germ- 

 cells. 



That the chromosomes supply the mechanism through 

 which inherited characters are passed on through successive 

 generations seems to be well established by the facts that 

 we have considered ; the linkage of various factors in the 

 same chromosome affords strong support for this view. It 

 is interesting to notice that further evidence has been 

 deduced from exceptions to the ordinary linkage phenomena 

 that have been observed by students of heredity in many 

 groups of animals, and notably by Morgan and his colleagues 

 in Drosophila (1922). They beheve that they have suc- 

 ceeded in locating the factors for a large number of varying 

 characters in one or other of the four chromosomes in the 

 gametes of this fly, and finding that the inheritance of these 

 characters is not always according to expectation — for 



, AbcD. ^ ABCD 



example -57^7 mstead of , , — they conclude that, when 



the chromosomes are paired for the reducing division and 

 more or less closely twisted or looped in certain regions of 

 their length, there may be a fracture and exchange of parts 

 between the two chromosomes of a pair ; the frequency of 

 this occurrence, known as ** crossing over," can be observed, 

 and from subsequent calculations the investigators believe 

 that they can '' map " the chromosomes so as to determine 

 approximately how the factors of the various characters are 

 arranged along their length. 



From the preceding statements and discussion one 



