THE BREAKDOWN OF CONTINUITY 



be brought about but even now we can reach a number of plausible 

 conclusions about ancestry from chromosome numbers. 



When we come to apply these rules we fmd that an enormous 

 variation exists in the stability of chromosome numbers in relation 

 to the external form in different groups of plants and animals. In 

 the flowering plants, where we can compare the numbers of some 

 10,000 species, the chief source of this variation becomes clear. In 

 the shorter-lived herbaceous plants chromosome numbers usually 

 vary within genera; in Crepisand Crocus^ for example, every haploid 

 number occurs between 3 and 18. In the longer-lived woody plants, 

 on the contrary, they remain constant sometimes for whole tribes 

 and families. Take the Pomoideae with their constant 17 chromo- 

 somes. This group presumably arose by an uneven or secondary 

 polyploidy from a section of the Rosacea^ with 7 chromosomes 

 in the Eocene period, since when no change save a renewed 

 polyploidy has occurred and established itself, and very little even 

 of that. It is, therefore, to the woody flowering plants (which 

 themselves must have arisen at different times from herbaceous 

 plants) that we can make the most far-reaching conjectures from 

 chromosome numbers with regard to descent and relationship. 



The diagram (Fig. 82) shows how these principles work, in the 

 first degree by polyploidy, in the second by losses and gains in 

 a polyploid, and in the third by losses and gains in diploids. It 

 shows 7 as the common ancestral chromosome number of 

 flowering plants. From this origin 8, 9 and an increasing series have 

 arisen on only a few occasions, whereas 14, with its diminishing 

 series, has arisen very frequently. In this scries 12 has often been 

 stabilized and from its addition to 7, 19 has appeared several times. 



One remarkable instance of this last step in the Magnoliales 

 requires special comment. If, as is customary, we divide the genera 

 with 19 chromosomes among 3 families we are implying that this 

 number arose from the union of 12 and 7 on three occasions. Thus 

 the morphologist's subdivisions of this group seem to be in conflict 

 with the probabilities of chromosome evolution, a conflict which 

 further study will readily resolve. 



These conclusions are at the moment necessarily conjectural. But 

 they enable us, by means of genetic theory and cytological tech- 

 nique, to join hands with the equally conjectural limits of systematics 



324 



