168 GENETICS: STURTEVANT, BRIDGES, AND MORGAN 
give a crossing over percentage of 35% or more no increase at all results. 
A summary of these facts extracted from my earlier paper is given below. 
The percentage of increase over the control is given. Only those data which 
were collected at the same time are entirely comparable with each other. 
As a whole this table shows conclusively that small percentages of crossing 
over are increased markedly by high or low temperatures, while larger per- 
centages show a much less significant increase. For instance, as a result of 
exposure of the Fi females to 32°C. (no. 7 in the table) the crossing over value 
between black and purple was increased from 6 in the control to 15.7 — an 
increase of 162% — while purple-curved in the same experiment showed an 
increase from 19.6 to 26.5 or only 35%. In no. 9 in the table it is shown that 
while the black-curved value was increased by 58% as a result of a tem- 
perature of 31.5° C, the value of the star to black region (38.4 units) not only 
did not increase, but actually showed a slight decrease. The amount of the 
increase is in all cases related to the crossover value involved. The last 
named case was explained in my former paper as follows (p. 157): 
The first brood data show very clearly that while the blackcurved region of the chromo- 
some shows an unquestionable increase of more than 50%, no increase at all is registered in 
the test between star and black. This can mean only that with such long distances any 
increase in the actual amount of single crossing over is compensated by a similar increase in 
double crossing over, and thus no increase at all appears in the percentage registered by the 
count. 
The data show that the percentage of increase caused by high or low 
temperature is roughly in inverse proportion to the size of the crossover value 
involved. On Castle's three dimensional scheme these facts necessitate the 
view that long chromosomal 'distances' are less affected by temperature 
than are short ones. On the hypothesis of linear arrangement this relation 
is consistently explained by the assumption that the amount of double 
crossing over is increased by high and by low temperatures. 
1 Plough, H. H., /. Exp. Zool., 24, 1917, (147). 
2 Bridges, C. B., Ibid., 19, 1915, (1). 
. THE SPATIAL RELATIONS OF GENES 
By A. H. Sturtevant, C. B. Bridges, and T. H. Morgan 
Columbia University and Carnegie Institution of Washington 
Communicated April 11, 1919 
Castle^ has proposed an arrangement of Hnked genes in three dimensions, 
based on the assumption that the distance between any two loci is exactly 
proportional to the observed crossover value. He believes that this system 
gives a better agreement between observation and expectation than does the 
hypothesis ot a strictly linear arrangement that we have developed. 
