APPENDIX 349 



elements participate, in an entirely free and equal arrangement, by 

 which it is only the diflPerentiating ones which nmtiially sei)arate 

 themselves. In this way the production would be rendered i)o.ssible 

 of as many sorts of egg and pollen cells as there are combinations 

 possible of the formative elements. 



The attribution attempted here of the essential difference in the 

 development of hybrids to a permanent or temporanj union of the 

 differing cell elements can, of course, only claim the vahie of an 

 hypothesis for which the lack of definite data offers a wide scope. 

 Some justification of the opinion expressed lies in the evidence 

 afforded by Pisum that the behaviour of each pair of difl'ercntiating 

 characters in hybrid union is independent of the other difi"erences 

 between the two original plants, and, further, that the hybrid i)ro- 

 duces just so many kinds of egg and pollen cells as there are possible 

 constant combination forms. The differentiating characters of 

 two plants can finally, however, only depend upon differences in 

 the composition and grouping of the elements which exist in 

 the foundation-cells [fertilised ova] of the same in vital inter- 

 action.^ 



Even the validity of the law formulated for Pisum requires still 

 to be confirmed, and a repetition of the more important experiments 

 is consequently much to be desired, that, for instance, relating to 

 the composition of the hybrid fertilising cells. A differential [ele- 

 ment] may easily escape the single observer,^ which although at the 

 outset may appear to be unimportant, may yet accumulate to such 

 an extent that it must not be ignored in the total result. Whether 

 the variable hybrids of other plant species observe an entire agree- 

 ment must also be first decided experimentally. In the meantime 

 we may assume that in material points an essential difference can 

 scarcely occur, since the unity in the developmental plan of organic 

 life is beyond question. 



In conclusion, the experiments carried out by Kolreuter, Gartner, 

 and others with respect to the transformation of one species into 

 another by artificial fertilisation merit special mention. Particular 

 importance has been attached to these ex])erimcnts and (liirtner 

 reckons them among '* the most diflScult of all in hyl)ridisation." 



If a species A is to be transformed into a species B, both must be 

 united by fertilisation and the resulting hybrids then be fertilised 



1 " Welche in den Grundzellen derselben in lehendiger Wechselwirkung stchen." 



2 *' Dem einzelnen Beobachter kann leicht ein Dijjcrenziale entgchcn." 



