48 BULLETIN 1164, U. S. DEPARTMENT OF AGRICULTURE. 
of characters was worked out on a Pima population of considerable 
size, with the result that a positive and significant correlation was 
detected. For these 22 examples of coherence the coefficients of cor- 
relation are uniformly small, the largest having been only 0.277 ± 0.046. 
This would indicate, on the assumption of genetic correlation, that 
the linkage is weak and that there is a high percentage of crossing 
over. 
It may be objected that the product-moment method of deter- 
mining the coefficient of correlation is inapplicable to characters 
in respect to which there was segregation in definite ratios, these 
having been petal spot, anther color, and midlock furrow index. 
These three characters showed no significant fourfold correlations 
with one another. In order to ascertain whether there were other 
correlations involving these characters which had not been revealed 
by the product-moment method, the F 2 population of 180 individuals 
was divided into two groups for petal spot, anther color, and midlock 
furrow, respectively, the basis of the groupings being indicated in 
the discussion of Mendelian segregation (pp. 21 to 27), and the means 
of the larger and of the smaller group were computed for each of the 
remaining characters. In every case where the means of the two 
groups differed significantly the correlation was worked out by the 
biserial method. This procedure subtracted none from the list of 
significant correlations as given in Table 12, and added the following: 
Petal spot and petiole hairiness, coefficient of biserial cor- 
relation -0.294±0.063 
Petal spot and lint index, coefficient of biserial correlation. — .283 ± .064 
Midlock furrow and leaf extension, coefficient of biserial 
correlation - ,208± .061 
The correlations of petal spot with petiole hairiness and with lint 
index are in the nature of coherences, while that of midlock furrow 
with leaf extension index is a disherence. 
Coherence of characters in cotton hybrids has been noticed exten- 
sively in the writings of O. F. Cook (11, 12, 13, 15), who defines 
coherence as — 
A condition in which characters derived from one parent remain together in ex- 
pression instead of being expressed in chance combinations as in Mendelian hvbrids 
(12, p. 30). 
Combinations in which the parental relations of the characters are 
reversed are regarded by Cook as being associated with infertility, for 
he remarks — 
Plants with these incongruous combinations of characters are generally infertile 
and sometimes completely sterile (12, p. 29). 
In an earlier publication (11, p. 17) Cook stated: 
Incongruities such as flowers of upland shape with the darker Egyptian color are 
rare and have been found thus far only on plants which are nearly sterile or otherwise 
definitely degenerate. 
Some idea of the frequency of occurrence of this incongruous com- 
bination is given in another paper (13, p. 53), the statement being 
that — 
A few hybrid plants, perhaps half a dozen out of as many thousands, have been 
found with the incongruous combination of Egyptian color with upland form, but 
these individuals were infertile and abnormal in other respects. The combination of 
white petals with Egyptian characters is less incongruous and much more frequent, 
in some Egyptian fields about one plant in a hundred. Some of the white-flowered 
