Chap. V. ANALOGOUS VARIATIONS. I59 



stems, or roots as commonly called, of the Swedish turnip and 

 Ruta-ba^'a, })hmts wliich several botanists rank as varieties 

 produced by cultivation from a common parent; if this be not 

 so, the case will then be one of analogous variation in two so- 

 called distinct species ; and to these a third may be added, 

 namely, the common turnip. AccoriUng to the ordinary view 

 of each species having be(?n independently created, we should 

 have to attribute this similarity in the enlarged stems of these 

 three plants, not to the vera causa of community of descent, 

 and a consequent tendency to vary in a like manner, but to 

 three separate yet closely-related acts of creation. Many simi- 

 lar cases of analogous variation have been observed by Naudin 

 in the great gourd-family, and by various authors in oiu* cereals. 

 Similar cases occurring with insects under their natural con- 

 ditions have lately been discussed with much al)ility by Mr. 

 Walsh, who has grouped theni under his law of Equable Va- 

 riability. 



With pigeons, however, we have another case, namely, the 

 occasional appearance in all the breeds of slaty-blue birds with 

 two black bars on the v/ings, white loins, a bar at the end of 

 the tail, with the outer feathers externally edged near their 

 bases with white. As all these marks are characteristic of the 

 parent rock-pigeon, I presume that no one will doubt that this 

 is a case of reversion, and not of a new yet analogous variation 

 appearing in the several breeds. ^Ve may, I think, confidently 

 come to tliis conclusion, because, as we have seen, these colored 

 marks are eminently liable to appear in tlie crossed oflsining 

 of two distinct and differently-colored breeds; and in this case 

 there is nothing in the external conditions of life to cause tlie 

 reappearance of the slaty-blue, with the several marks, beyond 

 the influence of the mere act of crossing on the laws of inher- 

 itance. 



No doubt it is a very surprising fact that characters should 

 reappear after having lieen lost for many, probably for hun- 

 dreds of generations. But when a ])reed has been crossed only 

 once by some otlier breed, the offspring occasionally show a 

 tendency to revert in character to the foreign breed for many 

 generations — some say for a dozen or even a score of genera- 

 tions. After twelve general i(ms, the proportion of blood, to 

 use a common expression, of any one ancestor, is only 1 in 

 3,048 ; and yet, as we see, it is generally believed that a ten- 

 dency to reversion is retained by tliis very small projiortion of 

 foreign blood. In a breed which has not been crossed, but 



