'206 DR J. M. MACFARLANE ON THE 



times exhibit a slight divergence toward one parent. Again, I shall have to refer at 

 some length to the remarkable change of colour exhibited by the flowers of Dianthus 

 Grievei, from white on first opening to rich crimson or crimson-purple on fading. The 

 one parent, D. alpinns, shows scarcely any trace of such floral change, but among 

 the numerous varieties of D. barhatus in cultivation one exhibits the above peculiarity 

 in an equally or even more striking manner. 



Now, every varietal form inherits certain common specific peculiarities, and also the 

 points that stamp it as a variety, so that one would err in comparing the ordinary species 

 with the hybrid. But the very fact that varieties are often inconstant in their varietal 

 details, and do not hand these down in all cases so steadily as a marked species, are 

 reasons for our giving a certain latitude in comparison with the hybrid, but equally are 

 reasons for our desiring an exact knowledge of how far a specific form may vary. 



(c) Comparison of Similar Parts. — In my earlier investigations it was sometimes 

 found that a certain part or organ of a hybrid did not exhibit intermediate blending of 

 the structure of both parents, but a decided leaning to one. This was at first regarded 

 as an instance of variation from average hybridity. But more careful and exhaustive com- 

 parison showed that the apparently exceptional conditions arose from choice of material 

 that did not agree in age, position, or opportunities for growth. Thus I stated in the 

 Gardeners 1 Chronicle (April 1890) that while Saxifraga Aizoon had many stomata on its 

 upper leaf surface, and S. Geum had none, S. Andrewsii resembled the latter in this 

 respect. Now, I had expected to find some on the leaf chosen from the hybrid, which 

 was one of the lowest of an annual shoot, those of the parents being from the upper parts 

 of shoots. On returning to the matter more recently, it was found that the closely inter- 

 mediate character of the hybrid was established when leaves of the same relative position and 

 age were chosen. Thus, since S. Aizoon produces on the average twenty-five leaves annually, 

 the hybrid thirty-two, and S. Geum forty, if the tenth leaf from the base be chosen in the 

 first, we should select the fourteenth in the hybrid and the eighteenth in the other parent. 



The same principle of judicious selection of material must be applied not only in dealing 

 with large organs but also in minuter details, such as bundle elements, matrix cells, and 

 sclerenchyma, as well as starch grains, chloroplasts, and other cell products. 



(d) Available Limit for comparison of Parents with their Hybrid Progeny. — During the 

 last decade problems bearing on the relative potency of the male and female elements in 

 the development of an organism have been greatly discussed. The present investiga- 

 tion not only throws great light on these, but will enable us to compare more accurately 

 than hitherto the capabilities of each sex element. It is manifest, however, that when a 

 hybrid is the product of parents that are widely divergent in histological details the com- 

 parison will be easy, but when we attempt to compare a hybrid with two parents which 

 are regarded as species, but whose chief specific differences are those of colouring and size, 

 it is almost or quite impossible to detect microscopically any blending of parent characters, 

 even though these may occur. Some may demur to accepting conclusions drawn from 

 comparison of the hybrids of two parents that are even moderately removed from each 



