CHROMOSOMES AND HEREDITY — DARLINGTON 425 



sion at the chiasma provide for the time, place, and action of the event 

 inferred in Drosophila and of the result seen in the chiasma [12] , 



Experiments with nucleic acid starvation later indicated that the 

 nucleic acid component of the chromosomes was the means of develop- 

 ing their torsion. I therefore assumed it to have the structure not of 

 a straight column but of a spiral staircase [13], an assumption which 

 has been vindicated with beautiful precision by Watson and Crick 

 [ 14] . How the molecular spiral works in detail, however, is a question 

 we must ask later, when we have a more elaborate molecular model of 

 the paired chromosomes. 



What must be discussed now is the fact that this breakage, this 

 crossing-over, can occur at hundreds or thousands of different places 

 along the chromosome — indeed by one definition between any pair of 

 genes in the whole sequence. But in a particular pair in a particular 

 cell it occurs at only a few points, from one to a dozen ; and there are 

 conditions, even in Drosophila itself, where it seems to be almost fixed. 



This situation, in our experience of the statistics of causal relations, 

 seemed to be unique and significant. Its mere mechanics was easily 

 understood. The frequency and distribution of crossing-over are char- 

 acteristic of the organism. It can be regulated by the organism, by 

 its heredity. If the chromosomes that are going to pair are regularly 

 placed side by side in the nucleus, which sometimes happens, the 

 amount of twisting they develop is regularly distributed and hence 

 the crossing-over. If the chromosomes are irregularly placed, as they 

 usually are, the crossing-over will be irregular and uncertain, as it 

 usually is. 



Thus the irregularity of crossing-over, which gives the character- 

 istic variety of progeny in sexually reproducing organisms, is some- 

 thing controlled. Like the weather it shows uncertainty. But, like 

 the weather also, we can predict it so far as we can expect to predict it. 

 Its failure, as well as its normal conditions, show that it is a deter- 

 mined uncertainty. Indeed in asexual reproduction all uncertainty 

 can be removed, and frequently is removed. Its general survival 

 throughout the plant and animal world therefore shows that the un- 

 certainty of crossing-over is original, is organized, and is of adaptive 

 value. Through it, indeed, meiosis acts as a means of generating 

 micertainty [15]. 



To put the matter in another way : it is a paradox that the gene 

 which is an organ of determinacy in life exists by virtue of a process of 

 apparent indeterminacy. But when we examine it we find that the 

 indeterminacy is spurious. It has been put there (if I may diverge 

 from the present argmnent) by natural selection and for natural 

 selection. It has been put there as a necessary complement of the un- 

 certainty of the gene's mutation; together they produce adaptive 

 variation. 



