four different kinds very close to {95} that
actually found by experiment. It will be noticed that in the whole family
the purples are to the reds as 3 : 1, and the longs are also three times as
numerous as the rounds. The peculiarity of the case lies in the
distribution of these two characters with regard to one another. In some
way or other the factors for blue and for long pollen become linked
together in the cell divisions that give rise to the gametes, but the
linking is not complete. This holds good for all the four cases in which
repulsion between the factors occurs when one of the two factors is
introduced by each of the parents. _When both of the factors are brought
into the cross by the same parent we get coupling between them instead of
repulsion._ The phenomena of repulsion and coupling between separate
factors are intimately related, though hitherto we have not been able to
suggest why this should be so.

Nor for the present can we suggest why certain factors should be linked
together in the peculiar way that we have reason to suppose that they are
during the process of the formation of the gametes. Nevertheless the
phenomena are very definite, and it is not unlikely that a further study of
them may throw important light on the architecture of the living cell.

APPENDIX TO CHAPTER IX

As it is possible that some readers may care, in spite of its complexity,
to enter rather more fully into the peculiar phenomenon {96} of the
coupling of characters, I have brought together some further data in this
Appendix. In the case we have already considered, where the factors for
blue colour and long pollen are concerned, we have been led to suppose that
the gametes produced by the heterozygous plant are of the nature 7 BL : 1
Bl : 1 bL : 7 bl. Such a series of ovules fertilised by a similar series of
pollen grains will give a generation of the following composition:--

49 BBLL + 7 BBLl + 7 BbLL + 49 BbLl
+ 7 BBLl + 7 BbLL + BbLl
+ BbLl
+ 49 BbLl
\---------------------------------/
177 purple, long

+ BBll + 7 Bbll + bbLL + 7 bbLl + 49 bbll
+ 7 Bbll + 7 bbLl
\-------------/ \-----------/ \-----/
15 purple, 15 red, 49 red,
round long round

and as this theoretical result fits closely with the actual figures
obtained by experiment we have reason for supposing t