252 CAUSES OF YAE I ABILITY. Chap. XXII 



abnormal structure ; but when she knew the nature of the 

 structure, she frequently suggested some fresh cause. The 

 belief in the power of the mother's imagination may perhaps 

 have arisen from the children of a second marriage resembling 

 the previous father, as certainly sometimes occurs, in accord- 

 ance with the facts given in the eleventh chapter. 



Crossing as a Cause of Variability. — In an early part of this 

 chapter it was stated that Pallas 33 and a few other naturalists 

 maintain that variability is wholly due to crossing. If this 

 means that new characters never spontaneously appear in our 

 domestic races, but that they are all directly derived from 

 certain aboriginal species, the doctrine is little less than 

 absurd ; for it implies that animals like Italian greyhounds, 

 pug-dogs, bull-dogs, pouter and fantail pigeons, &c, were able 

 to exist in a state of nature. But the doctrine may mean 

 something widely different, namely, that the crossing of 

 distinct species is the sole cause of the first appearance of new 

 characters, and that without this aid man could not have 

 formed his various breeds. As, however, new characters have 

 appeared in certain cases by bud-variation, we may conclude 

 with certainty that crossing is nut necessary for variability. 

 It is, moreover, certain that the breeds of various animals, 

 such as of the rabbit, pigeon, duck, &c, and the varieties of 

 several plants, are the modified descendants of a single wild 

 species. Nevertheless, it is probable that the crossing of 

 two forms, when one or both have long been domesticated 

 or cultivated, adds to the variability of the offspring, inde- 

 pendently of the commingling of the characters derived from 

 the two parent-forms ; and this implies that new characters 

 actually arise. But we must not forget the facts advanced 

 in the thirteenth chapter, which clearly prove that the act 

 of crossing often leads to the reappearance or reversion of 

 long-lost characters ; and in most cases it would be impossible 

 to distinguish between the reappearance of ancient characters 

 an I the first appearance of absolutely new characters. Practi- 

 cally, whether new or old, they would be new to the breed iD 

 which the}' reappeared. 



33 • Act Acad. St. Petersburg,' 1760. part ii. p. 84, &c 



