K. — BOTANY. 175 



combinations seen in the parents, the two smaller categories are those 

 representing the new combinations of one paternal with one maternal 

 characteristic. In the Sweet Pea several characters are linked in this 

 manner, viz. : flower colour with pollen shape, flower colom' with 

 form of standard, pollen shape with form of standard, colour of leaf 

 axil with functioning capacity of the anthers. If in these cases the 

 cross happens to be made in such a way that the two dominant 

 characters are brought in one from each side of the pedigree instead 

 of both being contributed by one parent, we get again a result in which 

 the two parental combinations occur more frequently, the two recom- 

 binations or ' crossovers ' less often than we should expect. In the 

 first case the two characters appear to hang together in descent to a 

 certain extent but not completely, in the latter similarly to repel each 

 other. This type of relationship has been found to be of very general 

 occurrence. The linked characters do not otherwise appear to be con- 

 nected in any way that we can trace, and we therefore conclude that 

 the explanation must be sought in the mechanism of distribution. Two 

 main theories having this fundamental principle as their basis but 

 otherwise distinct have been put forward, and are usually refen-ed to 

 as the reduplication and the chromosome view respectively. The 

 reduplication view, proposed by Bateson and Punnett,' rests on the idea 

 that segregation of factors need not necessarily occur simultaneously 

 at a particular cell division. The number of divisions follow- ing the 

 segregation of some factors being assumed to be greater than those 

 occurring in the case of others, there would naturally result a larger 

 number of gametes carrying some factorial combinations and fewer 

 carrying others. If this differential process is conceived as occurring 

 in an orderly manner it would enable us to account for the facts 

 obsei^ved. We could imagine how it came about that gametic ratios 

 such as 3 : 1 : 1 : 3 . 7 : 1 : 1 : 7, 15 : 1 : 1 : 15, and so on arose giving the 

 series of linkages observed. It has, however, to be said that we cannot 

 say why segregation should be successive nor at what moments, on 

 this view, it must be presumed to occur. On the other hand, .the 

 conceptions embodied in the chromosome hypothesis as formulated by 

 Morgan and his fellow-workers' are, in these respects, quite precise. 

 They are built around one cardinal event in the life cycle of animals 

 and plants (some of the lowest forms excepted), viz. : the peculiar tvpe 

 of cell division at which the number of chromosomes is reduced to 

 half that to be found during the period of the life cycle extending 

 backwards from this moment to the previous act of fertilisation. In 

 the large number of cases already investigated the chromosome number 

 has been found as a rule to be the same at each division of the somatic 

 cells. "We can, in fact, take it as established that it is ordinarily con- 

 stant for the species. These observations lend strong support to the 

 view that the chromosomes are persistent structures ; that is to say, 

 that the chromatin tangle of the resting nucleus, whether actually 

 composed of one continuous thread or not, becomes resolved into 



* Proc. Roy. Soc, 1911. 



' 7'he Mechanism of Meiidelian Heredity (Morgan, SUirtewant, Hnller. 

 Bridges), 1915. 



