Chap. XV. OF UNIFOKMITY OF CHAEACTEE. 65 



" wards the keepers frequently caught cross-bred rats, at 

 " first half-breds, afterwards with less of the character of the 

 " snake-rat, till at length all traces of it disappeared. 8 On 

 the other hand, in some parts of London, especially near the 

 docks, where fresh rats are frequently imported, an endless 

 variety of intermediate forms may be found between the 

 brown, black, and snake rat, which are all three usually 

 ranked as distinct, species. 



How many generations are necessary for one species or race 

 to absorb another by repeated crosses has often been dis- 

 cussed; 9 and the requisite number has probably been much 

 exaggerated. Some writers have maintained that a dozen 

 or score, or even more generations, are necessary ; but this in 

 itself is improbable, for in the tenth generation there would 

 be only 1-1 024th part of foreign blood in the offspring. 

 Gartner found, 10 that with plants, one species could be made 

 to absorb another in from three to five generations, and he 

 believes that this could always be effected in from six to 

 seven generations. In one instance, however, Kolreuter 11 

 speaks of the offspring of Mirabilis vulgaris, crossed during 

 eight successive generations by M. longiflora, as resembling 

 this latter sjoecies so closely, that the most scrupulous 

 observer could detect " vix aliquam notabilem differentiam " 

 or, as he says, he succeeded, " ad plenariam fere transmuta- 

 tionem." But this expression shows that the act of absorp- 

 tion was not even then absolutety complete, though these 

 crossed plants contained only the l-256th part of M. vulgaris. 

 The conclusions of such accurate observers as Gartner and 

 Kolreuter are of far higher worth than those made without 

 scientific aim by breeders. The most precise account which 

 I have met with is given by Stonehenge, 12 and is illustrated 

 by photographs. Mr. Hanley crossed a. greyhound bitch with 

 a bulldog ; the offspring in each succeeding generation being 

 recrossed with first-rate greyhounds. As Stonehenge remarks, 



8 Mr. S. J. Salter, ' Journal Linn. crosses. Dr. P. Lucas, ' L'Heredite 

 Soc.,' vo„ vi., 1862, p. 71. Nat.,' torn. ii. p. 308. 



9 Sturm, 'Ueber Racen, &c.,' 1825, 10 < Bastarderzeugung,' s. 463, 470, 

 s. 107. Bronn, ' Geschichte der Na- u ' Nova Acta Petrop.,' 1794, p, 

 tur,' b. ii. s. 170, gives a table of the 393 : see also previous volume, 

 proportions of blood after successive 12 ' The Dog,' 1867, pp. 179-184. 



