GENETICS IN THE SERVICE OF MAN — GLASS 305 



been used to obtain strains with a constant high sugar content in the 

 stalk, and now hybrid corn with sweet stalks is ready to be used, either 

 for the human sweet tooth or as sihige for our horses and cattle, which 

 need sugar too. A short-er corn pLint that would stop growing at less 

 than 6 feet in height would clearly be invaluable to the seed growers 

 of hybrid corn, who find some difficulty in detasseling, by hand, corn 

 plants that grow 12 feet tall. The desired mutation was found, and 

 now hybrid corn that is just as productive as formerly, but grows to 

 only 6 feet, is available. 



The third evolutionary factor is genetic intermixture, certain possi- 

 bilities of which have already been indicated in what has been said 

 about hybrid corn. Intermixture may, however, be extended to wider 

 limits, to encompass crosses between different geographic races or even 

 different species. The latter have evolved to a point where the hybrids 

 between them are commonly highly sterile — witness the mule. Yet 

 just here, by an odd chance, there emerges the very mechanism that 

 has enabled the geneticist to create his first true new species. For if 

 in some way the chromosomes of a sterile hybrid can be doubled, its 

 self- fertility is often completely restored, although it remains infertile 

 when crosses are made with either of the parent species. If, for 

 example, one could double the chromosomes of the mule, the latter 

 would have two sets of hoi'se chromosomes and two sets of ass chromo- 

 somes. Hybrid sterility is often due to the inability of the chromo- 

 somes of different species to pair with one another during the forma- 

 tion of the sex cells ; but after doubling, one set of horse chromosomes 

 could pair with the other and likewise for the ass chromosomes, so 

 that each e^g cell or each sperm cell would possess when mature a full 

 set for both kinds. No one has yet succeeded in doing this to a mule, 

 or in breeding two mules together afterward, but exactly this feat has 

 been accomplished a number of times in the plant world. 



The first and most famous instance was performed by a Russian 

 geneticist, G. D. Karpechenko, in 1927. Karpechenko crossed two 

 different genera, the radish (Raphanus) with the cabbage {Brassicd), 

 and obtained a sterile hybrid. He then succeeded, with some difficulty, 

 in getting the chromosomes to double, following which he could self- 

 pollinate the hybrid and obtain in the next generation a perfectly 

 fertile form which he named Raphanobrassica and which, according 

 to the same etymological principle, should in English be called by the 

 common name of "rabbage." Since it could be crossed with the orig- 

 inal radish or cabbage parent species only with a resultant almost- 

 complete breakdown of fertility, Karpechenko rightly regarded this 

 as a new species, tlie first man-made one in history. 



But unfortunately for Karpechetdco, the new rabbage species com- 

 bined the prickly inedible leaves of the radish with the miserable root 

 of a cabbage. Although he received worldwide fame among geneti- 



