EVOLUTION— CONKLIN" 213 



Nearly a score of new species of plants, having all the character- 

 istics of true Linnaean species, have been artificially produced by 

 hybridization or experimental operations with consequent changes in 

 chromosome numbers and associations. These new species are fertile 

 inter se, but are sometimes sterile when crossed with either one or 

 both of the parent species, thus fulfilling the strictest definition of 

 true species as laid down by many systematists. Thus Goodspeed 

 and Clausen (1925) crossed two species of tobacco plants, namely 

 Nicotlana glutinosa with 12 haploid chromosomes and N. tcibaeum 

 with 24. The first hybrid generation normally had 36 somatic chrom- 

 osomes, and they were generally sterile, but one partially fertile 

 hybrid produced second generation plants, one of which was remark- 

 ably large and robust and was found to have 72 somatic chromo- 

 somes; that is, it was a tetraploid or gigas form. This plant bred 

 true but was sterile when back crossed to one of the parent species ; 

 it has been named N. digluta (Clausen 1928). 



Another case of the production of a true synthetic species by hy- 

 bridization and subsequent doubling of the number of chromosomes 

 was described by Newton and Pellew (1929) ; two distinct species of 

 primrose, P. verticellata and P. fj(yi^%b<u,nda crossed and produced a 

 sterile hybrid; this was propagated vegetatively for several years 

 when it suddenly produced a fertile shoot by bud transformation 

 which bore normal seeds, and from these arose a new and fertile 

 species, P. kewensis^ with a tetraploid number of chromosomes. 



Lindstrom (1932) cut off the tops of young tomato plants of 

 the species Lyco'persioiMn fiinpineMlfoliwm^ and in the callus that 

 formed chromosome doubling took place in some of the cells, and 

 from these cells some tetraploid sprouts arose and bore fruit and 

 seeds. These were highly fertile and have produced plants so dif- 

 ferent from the original stock that they should be classed as a new 

 species, especially as they are cross-sterile with the parent species. 



Another new species produced by hybridization is the pink chest- 

 nut, Aesculus carnea^ from a cross between A. hifpocastanum and 

 A. pavia, the former with 20 small chromosomes, the latter with 20 

 large ones, while the new species has 20 large and 20 small chromo- 

 somes, or 40 in all (Hurst, 1932). 



Still more remarkable are the results of crossing distinct genera 

 of plants such as the common radish, Raphanus sativus, and the 

 cabbage, Brassica oleracea, each with 9 haploid chromosomes lead- 

 ing to the production of a new tetraploid genus Raphanohrassica 

 with 3G chromosomes (Karpechenko, 1929) ; or the formation of a 

 new genus TriticaJe by crossing wheat, Triticmn vulgare, and rye, 

 Secale cereale (Levitsky and Benetzkaia, 1929). 



All of the preceding cases have to do with the production of 

 new mutants or true species by changes in the numbers and asso- 



