in Hybridisation 373 



developmental series it could not in fact be otherwise. 

 For seven differentiating characters, for instance, among 

 more than 16,000 individuals offspring of the hybrids- 

 each of the two original species would occur only once. It 

 is therefore hardly possible that these should appear at all 

 among a small number of experimental plants ; with some 

 probability, however, we might reckon upon the appearance 

 in the series of a few forms which approach them. 



We meet with an essential difference in those hybrids 

 which remain constant in their progeny and propagate 

 themselves as truly as the pure species. According to 

 Gartner, to this class belong the remarkably fertile hybrids 

 Aquilegia atropurpurea canadensis, Lavaterapseudolbia thu- 

 ringiaca, Geum urbano-rivale, and some Dianthus hybrids ; 

 and, according to Wichura, the hybrids of the Willow 

 family. For the history of the evolution of plants this 

 circumstance is of special importance, since constant hybrids 

 acquire the status of new species. The correctness of the 

 facts is guaranteed by eminent observers, and cannot be 

 doubted. Gartner had an opportunity of following up 

 Dianthus Armeria deltoides to the tenth generation, since 

 it regularly propagated itself in the garden. 



With Pisum it was shown by experiment that the 

 hybrids form egg and pollen cells of different kinds, and that 

 herein lies the reason of the variability of their offspring. 

 In other hybrids, likewise, whose offspring behave similarly 

 we may assume a like cause ; for those, on the other hand, 

 which remain constant the assumption appears justifiable 

 that their reproductive cells are all alike and agree with the 

 foundation-cell [fertilised ovum] of the hybrid. In the 

 opinion of renowned physiologists, for the purpose of 

 propagation one pollen cell and one egg cell unite in 

 Phanerogams^ into a single cell, which is capable by 



* In Pisum it is placed beyond doubt that for the formation of the 

 new embryo a perfect union of the elements of both reproductive cells must 

 take place. How could we otherwise explain that among the offspring of 

 the hybrids both original types reappear in equal numbers and with all 

 their peculiarities ? If the influence of the egg cell upon the pollen cell 

 were only external, if it fulfilled the role of a nurse only, then the result 

 of each artificial fertilisation could be no other than that the developed 

 hybrid should exactly resemble the pollen parent, or at any rate do so very 

 closely. This the experiments so far have in no wise confirmed. An 



