670 
GENETICS: BABCOCK AND COLLINS Proc. N. A. S. 
factors, we then naturally * wonder just what part "distance between two 
genes" on a chromosome map may play in determining linkage values. 
Our current view is that "the percentage of cases in which two linked 
genes separate (amount of crossing over between them) is necessarily 
proportional to the distance between these genes, — other things being 
equal," i. e., under ordinary circumstances and in the absence of unusual 
factors or environmental conditions which geneticists recognize. But 
evidently under ordinary circumstances, the percentage of crossing over is a 
variable which is determined by the different possible combinations of mul- 
tiple modifying factors ; hence the percentage of crossing over cannot be pro- 
portional to the distance if the distance remain constant. For example in 
Series B we find 6% crossing over, and so we should conclude that the dis- 
tance is less than one-fifth of what it originally was before we began selection. 
To maintain our original position, we must conclude that the percentage 
of crossing over and the distance are correlated variables, if the proportion 
is to remain reasonably constant. The dilemma will hardly aid us in 
determining what had happened to almost all of the distance and the 
genes between 0 and 33 in Series A and A' , where crossing over was prac- 
tically eliminated. In view of these considerations it would perhaps be 
simpler to conclude that linkage is not a function of distance, i. e., crossing 
over is not necessarily proportional to distance. The distance between 
two genes may remain fairly constant, but the amount of crossing over 
depends upon numerous hereditary factors. 
^ Paper No. 14 from the Laboratory of Genetics, Illinois Agricultural Experiment 
Station . 
2 In giving crossover values, I shall put the data in the following form throughout 
this paper — crossovers: total = per cent of crossing over. Since the classes are 
always the same, repetition can be avoided. 
" I am indebted to Dr. E. Roberts and Mr. P. M. Woodworth for carrying this genera- 
tion in part. 
INTERSPECIFIC HYBRIDS IN CREPIS 
I. Crepis capillaris (L) Wallr. X C. tectorum L. 
By K. B. Babcock and J. L. Coluns 
Division of Ge;ne;tics, CoIvLBge; of Agricui^ture, University of Cai^ifornia 
Communicated by J. C. Merriam, September 22, 1920 
Interspecific hybrids, particularly those having different specific chromo- 
some numbers, have in the past figured conspicuously in establishing the 
now generally accepted principle of the individuality and continuity of 
the chromosomes. It is probable, therefore, that a study of the hereditary 
characters of species hybrids may give further information concerning the 
chromosomes as the carriers of the determiners of hereditary characters 
