On the Origin of Species in Flowering Plants

Abstract

THE origin of species is the origin of the difference between two species. The existence of a peculiar difference between good species, which does not exist between varieties, has long been realised by systematists and others. This essential difference expresses itself, in part, in flowering plants, in the abortion of pollen and ovules in the hybrids, which springs, in typical cases: (a) from a different distribution of genes between two or more chromosome pairs; so that vital genes are omitted after the reduction division in some or all of the groups of chromosomes of the F1 of the cross. An example of this is probably afforded by the crosses of certain species of Stizolobium (Mucuna) (Zeitsch. f. ind. Abst. u. Vererb., pp. 303–342, 1914), two non-homologous chromosomes appearing to have interchanged genes in one of the species. In the second case (b) the attraction between chromosomes which should be homologous fails in the reduction divisions of the F1 plant, so that non-conjunction (Journal of Genetics, 1925) takes place with regard to one or more pairs. Thus there is an absence of vital chromosomes or the presence of extra chromosomes in some of the pollen and ovules. This is illustrated by the two cannas “Austria” and “Italia”, which belong no doubt, as their history states, to the F1 of species crosses. In “Austria”, for example, the 18 single chromosomes show only 1–3 pairs at the reduction divisions. All the other chromosomes segregate at random, so that no more than one pollen-grain or embryo-sac out of thousands contains only the 9 chromosomes from one parent, and survives. In the cross of radish and cabbage, as Karpechenko (Journal of Genetics, vol. 14, pp. 375–396, 1924) has lately shown, chromosome conjugation is absent. This has also been shown in certain Digitalis crosses.