250 



are harmful to their carriers, as might 

 be expected from a bhnd interference 

 with the dehcately balanced mecha- 

 nisms of life. Most mutants have a 

 lower viability and a poorer fecundity 

 than the t\pcs they are derived from. 

 Yet this is not always so. Sometimes a 

 new mutant type may be poorer than 

 the original type under the existing 

 conditions of life, but may prove itself 

 superior when these are altered. Flies 

 dependent on garbage pails do better in 

 the city of Baltimore if they have 

 wings, but on the storm-swept island 

 of Kerguelen in the southern Indian 

 Ocean the only flics to be found creep 

 about without wings, or with little 

 stubbv vestiges of wings. Natural selec- 

 tion, as Darwin pointed out, deter- 

 mines the differential sur\'ival of various 

 hcrcditan' types, and natural selection 

 is but a name for the complex combi- 

 nation of conditions under which each 

 population lives and reproduces, and 

 which is different, at least somewhat 

 different, in every other time and place. 

 The third evolutionan,' factor is 

 genetic intermixture, certain possibil- 

 ities of which have already been indi- 

 cated in what has been said about hy- 

 brid com. Intermixture may, however, 

 be extended to wider limits, to encom- 

 pass crosses between different geo- 

 graphic races of 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 ver\' mechanism that 

 has enabled the geneticist to create his 

 first true new species. For if in some 

 way the chromosomes of a sterile hy- 

 brid can be doubled, its self-fertility is 

 often completely restored, although it 

 remains infertile when crosses are made 

 with cither of the parent species. If, for 

 example, one could double the chro- 

 mosomes of the mule, the latter would 

 ha\e two sets of horse chromosomes 

 and two sets of ass chromosomes. Ily- 



Cenetics and plant breeding 



brid sterility is often due to the inabil- 

 ity of the chromosomes of different 

 species to pair with one another during 

 the formation of the sex cells; but after 

 doubling, one set of horse chromo- 

 somes could pair with the other and 

 likewise for the ass chromosomes, so 

 that each egg cell or each sperm cell 

 would possess when mature a full set 

 for both kinds. No one has yet suc- 

 ceeded in doing this to a mule, or in 

 breeding two mules together after- 

 wards, but exactly this feat has been 

 accomplished a number of times in the 

 plant world. 



Tlie first and most famous instance 

 was performed by a Russian geneticist, 

 G. D. Karpechenko, in 1927. Karpe- 

 chenko crossed two different genera, 

 the radish (Raphanus) with the cab- 

 bage (Brassica), and obtained a sterile 

 h)'brid. 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 Raphanobras- 

 sica and which, according to the same 

 etvmological principle, should in Eng- 

 lish be called by the common name of 

 "rabbage." Since it could be crossed 

 with the original radish or cabbage par- 

 ent species only with a resultant 

 almost-complete breakdown of fertil- 

 ity, Karpechenko rightly regarded this 

 as a new species, the first man-made 

 one in history. 



But unfortunately for Karpechenko, 

 the new rabbage species combined the 

 prickly inedible leaves of the radish 

 with the miserable root of a cabbage. 

 Although he received worldwide fame 

 among geneticists for his feat, it was 

 scarcely an achievement to impress the 

 makers of agricultural five-year plans. 

 Karpechenko was later liquidated. The 

 method is nonetheless one of great 

 promise, for in some instances the valu- 

 able characteristics of two species may 

 thus be combined in a single new one; 



