Chap. XXVIII. CONCLUDING REMARKS. 413 



and the high antiquity of man, as a breeder of animals, haa 

 only recently become kuown. Most naturalists, however, 

 freely admit that our various breeds, however dissimilar, 

 are descended from a single stock, although they do not 

 know much about the art of breeding, cannot show the 

 connecting links, nor say where and when the breeds arose. 

 Yet these same naturalists declare, with an air of philo- 

 sophical caution, that they will never admit that one natural 

 species has given birth to another until they behold all the 

 transitional steps. Fanciers use exactly the same language 

 with respect to domestic breeds ; thus, an author of an excellent 

 treatise on pigeons says he will never allow that the carrier 

 and fantail are the descendants of the wild rock-pigeon, 

 until the transitions have " actually been observed, and can 

 " be repeated whenever man chooses to set about the task." 

 No doubt it is difficult to realise that slight changes added 

 up during long centuries can produce such great results; 

 but he who wishes to understand the origin of domestic breeds 

 or of natural species must overcome this difficulty. 



The causes which excite and the laws which govern varia- 

 bility have been discussed so lately, that I need here only 

 enumerate the leading points. As domesticated organisms are 

 much more liable to slight deviations of structure and to mon- 

 strosities than species living under their natural conditions, 

 and as widely-ranging species genei ally vary more than those 

 which inhabit restricted areas, we may infer that variability 

 mainly depends on changed conditions of life. We must not 

 overlook the effects of the unequal combination of the charac- 

 ters derived from both parents, or reversion to former pro- 

 genitors. Changed conditions have an especial tendency to 

 render the reproductive organs more or less impotent, as 

 shown in the chapter devoted to this subject ; and these 

 organs consequently often fail to transmit faithfully the 

 parental characters. Changed conditions also act directly 

 and definitely on the organisation, so that all or nearly all 

 the individuals of the same species thus exposed become 

 modified in the same manner ; but why this or that part is 

 especially affected we can seldom or ever say. In most 

 cases, however, a change in the conditions seems to act 



