88 ON CROSSING AS A CAUSE C hap. XV 



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 discussed ; • 

 and the requisite number has probably been much exagge- 

 rated. 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 will be only 

 l-1024th 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 



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* 



I 





instance, however, Kolreuter 11 speaks of the offspring of Mr a 

 bilis vulgaris, crossed during eight successive generations by M 



longiflora, as resembling this latter species so closely, that 



most scrupulous observer could detect " vix aliquam notabilem 

 differentiam ; "—he succeeded, as he says, " ad plenariam fere 

 transmutationem." But this expression shows that the act of 

 absorption was not even then absolutely complete, though these 



ed plants contained only the l-256th part of M. vuty 

 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 remarkable statement 

 which I have met with of the persistent endurance of the effects 

 of a single cross is given by Fleischmann, 12 who, in reference 

 to German sheep, says " that the original coarse sheep have 

 " 5500 fibres of wool on a square inch ; grades of the third or 

 " fourth Merino cross produced about 8000, the twentieth cross 

 " 27,000, the perfect pure Merino blood 40,000 to 48,000." So 

 in this case common German sheep crossed twenty times succes- 

 sively with Merinos have not by any means acquired wool as fine 

 as that of the pure breed. In all cases, the rate of absorption will 



9 Sturm, < Uebcr Bacen, &c.,' 1825, s. "> t Bastarderzeugung," s. 463, 470. 



107 : Bronn, ' Gescliicktc der Natur/ » 'Nova Acta Petrop.,' 1794, p. 393 : 



b. ii. s. 170, gives a table of the propor- see also previous volume, 

 tions of blood after successive crosses. 12 As quotec j j n t j ie < True Principles 



Dr. P. Lucas, ' lHeredite Nat.,' torn. ii. of Breeding,' by O. H. Macknight and 



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p. 308. 



Dr. H. Madden, 1865. p. 11. 









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