256 Fertilisation and Hybridisation 



character. And, on the whole, experience has so far 

 proven the reliabilty of these formulae for animals as well 

 as for plants. 



It would be too great a digression to consider here the 

 formulae themselves. We shall therefore leave the field 

 of the variety-hybrids, and turn to the hybrids between 

 different species, especially between allied elementary spe- 

 cies. 



In order to understand these we must get a clear idea 

 of the nature of the points of difference in this case, or in 

 other words, what is meant by relationship. Species orig- 

 inate from each other in a progressive way. The number 

 of the units in lower organisms is evidently only small, 

 and must gradually increase with progressing organiza- 

 tion. Every newly arising species contains at least one 

 more than the form from which it has arisen. Only in 

 this way can one imagine the progress of the entire plant 

 and animal world. 7 



It is indeed questionable whether the acquisition of a 

 single new unit, the increasing by one unit of the entire 

 stock, amounting to hundreds and thousands, would be 

 sufficient to make the impression of progress on us. The 



7 A quite different hypothesis is thinkable, as, for example, that 

 suggested by G. H. Shull, "The Significance of Latent Characters," 

 Science N. S., 25 : 792. 1907. 



"All the visible variations of the present plant and animal world 

 were once involved in some generalized form or forms, and the pro- 

 cess of differentiation pictures itself to us as a true process of evolu- 

 tion brought about by the change of individual character-determining 

 units from a dominant to a recessive state. This conception results 

 in an interesting paradox, namely the production of a new character 

 by the loss of an old unit." 



This hypothesis, however, as de Vries has pointed out, seems too 

 much like a revival of the old evolution theory as opposed to epi- 

 genesis. Tr. 



