112 GOOD FEOM CROSSING. Chap. XVIL 



their enormous roots/^ have been transmitted to tlieir progeny. 

 The result in all cases is probably in part cine to the saving of 

 nutriment and vital force through the sexual organs acting imper- 

 fectly or not at all, but more especially to the general law of good 

 being derived from a cross. For it deserves especial attention that 

 mongrel animals and plants, which are so far from being sterile that 

 their fertility is often actually augmented, have, as jDreviously 

 shown, their size, hardiness, and constitutional vigour generally 

 increased. It is not a little remarkable that an accession of vigour 

 and size should thus arise under the opi^osite contingencies of 

 increased and diminished fertility. 



It is a perfectly well ascertained fact^* that hybrids invariably 

 breed with either pure parent, and not rarely with a distinct species, 

 more readily than with one another. Herbert is inclined to explain 

 even this fact by the advantage derived from a cross ; but Gartner 

 more justly accounts for it by the pollen of the hybrid, and 

 probably its ovules, being in some degree vitiated, whereas the 

 pollen and ovules of both pure parents and of any third species are 

 sound. Nevertheless, there are some well-ascertained and "re- 

 markable facts, which, as we shall presently see, show that a cross 

 by itself undoubtedly tends to increase or re-establish the fertility 

 of hybrids. 



The same law, namely, that the crossed offspring both of varieties 

 and species are larger than the parent-forms, holds good in the 

 most striking manner with hybrid animals as well as with mongrels. 

 Mr. Bartlett, who has had such large experience says, " Among all 

 " hybrids of vertebrated animals there is a marked increase of size." 

 He then enumerates many cases with mammals, including monkeys, 

 and with various families of birds.^^ 



On certain Hermaplirodite Plants ivliich, eitlier normally or abnor- 

 mally, require to he fertilised hy pollen from a distinct individual 

 or species. 



The facts now to be given diifer from the foregoing, as 

 self-sterility is not here the result of long-continued close 

 interbreeding. These facts are, however, connected with our 

 present subject, because a cross with a distinct individual is 

 shown to be either necessary or advantageous. Dimorphic 

 and trimorphic plants, though they are hermaphrodites, must 

 be reciprocally crossed, one set of forms by the other, in order 

 to be fully fertile, and in some cases to be fertile in any degree. 



. «3 Kolreuter, < Nova Acta,' 1795, 430. 



p. 316. «^ Quoted by Dr. Murie, iu ' Proc. 



** Gartuer, ' Bastarderzeugung,' s. Zoolog. Soc.,' 1870, p. 40. 



