250 STERILITY OF HYBRIDS. [BOOK n. 



fixed characters, as well as those variations which even now 

 frequently arise and are nearly fixed characters, but not 

 absolutely so, and those which are more variable and very 

 subject to relapse in reproduction ? " 



The cause of the frequent sterility of mule plants is at 

 present unknown. Sometimes, indeed, a deficiency of pollen 

 may be assigned ; but in many cases there is no perceptible 

 difference in the healthiness of structure of the fertilising 

 organs of a mule plant and of its parents. Professor Henslow, 

 of Cambridge, in an excellent paper upon a hybrid Digitalis, 

 investigated anatomically the condition of the stamens and 

 pistil, both of his hybrid and its two parents, with great care 

 and skill. The result of his inquiry was, that no appreciable 

 difference could be detected. 



Dr. Herbert's views were the following : " I suspect that 

 it is by the nice adaptation of the juices of each individual 

 type to yield the exact proportion of what is wanted for the 

 pollen of its kind, that the Almighty has limited the races of 

 created things ; and that, wherever that adaptation is perfect, 

 a perfect offspring is produced. Where it is not perfect, an 

 inadequate or a weak fertilisation takes place. It is further 

 to be observed that there is frequently an imperfect hybrid 

 fertilisation, which can give life, but not sustain it well. 

 There are several crosses which I have repeatedly obtained, 

 but could not raise the plants to live for any length of time. 

 I obtained much good seed several years ago from Hibiscus 

 palustris by speciosus ; I sowed a little each year till it was 

 all gone ; the plants always sprouted, but I saved only one to 

 the third leaf, and it perished then. I have never raised 

 beyond the third or fourth leaf a cross between Rhododendron 

 ponticum and an orange Azalea, though I have saved two or 

 three through the first winter. My soil, however, is very 

 uncongenial to them, and under more favourable circum- 

 stances they would have been saved. From Rhodora 

 canadensis by Azalea pontica (sections of genus Rhododen- 

 dron), I saved ultimately only one out of more than a 

 hundred seedlings, and that became a vigorous plant. Such 

 crosses sometimes are a hundred times more delicate in their 



