256 CAUSES OF VARIABILITY. Chap. XXIL 



With respect to the part which, the reproductive system takes in 

 causing variability, we have seen in the eighteenth chapter that 

 even slight changes in the conditions of life have a remarkable 

 power in causing a greater or less degree of sterility. Hence it 

 seems not improbable that beings generated through a system so 

 easily affected should themselves be affected, or should fail to 

 inherit, or inherit in excess, characters proper to their parents. 

 We know that certain groups of organic beings, but with exceptions 

 in each group, have their reproductive systems much more easily 

 affected by changed conditions than other groups; for instance, 

 carnivorous birds more readily than carnivorous mammals, and 

 parrots more readily than pigeons ; and this fact harmonises with 

 the apparently capricious manner and degree in which various 

 groups of animals and plants vary under domestication. 



Kolreuter 47 was struck with the parallelism between the excessive 

 variability of hybrids when crossed and recrossed in various ways, — 

 these hybrids having their reproductive powers more or less affected, 

 ■ — and the variability of anciently cultivated plants. Max AYichura 4 * 

 has gone one step farther, and shows that with many of our highly 

 cultivated plants, such as the hyacinth, tulip, auricula, snapdragon, 

 potato, cabbage, &c, which there is no reason to believe have been 

 hybridised, the anthers contain many irregular pollen-grains in the 

 same state as in hybrids. He finds also in certain wild forms, the 

 same coincidence between the state of the pollen and a high degree 

 of variability, as in many species of Eubus; but in B. ccesius and 

 idceus. which are not highly variable species, the pollen is sound. 

 It is also notorious that many cultivated plants, such as the banana, 

 pine-apple, bread-fruit, and others previously mentioned, have their 

 reproductive organs so seriously affected as to be generally quite 

 sterile; and when they do yield seed, the seedlings, judging from 

 the large number of cultivated races which exist, must be variable 

 in an extreme degree. These facts indicate that there is some 

 relation between the state of the reproductive organs and a tendency 

 to variability; but we must not conclude that the relation is strict. 

 Although many of our highly cultivated plants may have their 

 pollen in a deteriorated condition, yet, as we have previously seen, 

 they yield more seeds, and our anciently domesticated animals are 

 more prolific, than the corresponding species in a state of nature. 

 The peacock is almost the only bird which is believed to be less 

 fertile under domestication than in its native state, and it has varied 

 in a remarkably small degree. From these considerations it would 

 seem that changes in the conditions of life lead either to sterility 

 or to variability, or to both ; and not that sterility induces variability. 

 On the whole it is probable that any cause affecting the organs of 



47 ' Dritte Fortsetzucg,* &c, 1766, Berkeley on the same subjeH, in 

 s 35. 'Journal oi Royal Hurt. Sue./ IK'ib, 



48 'Die Bastardbefruchtnng.' &c, p. 80. 

 186"., s. 92: see also the Rev. M. J. 



