76 THE PKINCIPLES OF HEREDITY 



able than that, in spite of plain, abundant, and conclusive 

 evidence, it should so long have been regarded as the cause of 

 progressive variations. 



131. Before attempting to estimate the function of bi- 

 parental reproduction in evolution it is necessary to return 

 for a space to the general subject of regressive variations. 

 Reversion can seldom be observed in such of the higher 

 plants and animals as have been slowly evolved under Natural 

 Selection ; not because it does not occur, but because it is 

 usually masked and slight. It is masked because such com- 

 plex beings seldom or never regress in all their characters at 

 once, and therefore such reversion as may occur in this or that 

 particular is associated with progressive variations in other 

 particulars. It is slight because, since such species have 

 evolved but slowly, reversion to a not very remote ancestor 

 does not usually result in any very appreciable change of 

 type. Thus, if, as regards any character, a man reverted to 

 his " mid-ancestor," or, in cases of exclusive inheritance, to any 

 particular ancestor, of a thousand years ago, no one would be 

 able to recognize to what the change of type was due. Not 

 only would the changes, as a rule, be too small to be noticed, 

 but the observer would need to have an impossibly exact 

 knowledge of the ancestral form. 



132. Sometimes, however, recognizable reversion does occur 

 in such beings. Thus a man may resemble the portrait of 

 some far-away ancestor, or the progeny of a pair of ordinary 

 horses may exhibit the zebra-like stripes of the ancestral 

 form. In other words, the progressive variations made 

 during hundreds or thousands of generations may be lapsed in 

 a single generation. It is not however among complex beings, 

 slowly evolved in every particular, that we shall find the most 

 convincing evidence. We must turn to animals and plants 

 that have undergone swift evolution in one or two particulars, 

 and this occurs, speaking generally, only under stringent 

 artificial selection. For Natural Selection, having care for 

 many characters, results in comparatively slow evolution ; but 

 artificial selection, having care for only one, or only a few 

 characters, results in much swifter evolution. Domesticated 

 plants and animals, as we have just seen, frequently revert to 

 or far towards the wild type. Again, if we take any prize 

 breed of domesticated animals or plants, and, after choosing 

 the finest specimens, henceforward breed indiscriminately 

 from their descendants, the cessation of selection is invariably 

 marked by a reversion towards the ancestral type which is 

 swift in proportion to the swiftness of the antecedent evolu- 



