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THE AMEBIC AX NATURALIST [Vol. XL V 



portance with that of the two parent species, for it will 

 take an equal part in producing descendants. Further- 

 more, crossing between the new hybrid forms and the 

 parents will go on indefinitely, producing a host of inter- 

 breeding races, and this population will constitute the 

 material for further evolution. 



I have been studying this process by means of cul- 

 tures from seeds of plants on the sand dunes near Liver- 

 pool, England, where biennis, Lcunarckiana and grandi- 

 flora have been interbreeding freely for at least a cen- 

 tury, producing a host of races (including several of the 

 mutants of 0. Lamarckiana), some of which split, but 

 many are found to breed true for at least two genera- 

 tions. It seems to me that the question to ask regarding 

 these races is not as to whether they have originated in 

 connection with crossing, for it is certain that they have 

 originated at least in the presence of conditions in which 

 crossing i s taking place in every generation. The ques- 

 tion is rather what relation (if any) these races bear to 

 the previous evolution of these forms, whether any of 

 them are progressive in the sense that they will supplant 

 the other races and become the progenitors of future 

 generations, to the exclusion of those which were for- 

 merly progenitors. In other words, are the new races 

 progressive in the sense that they show some further de- 

 velopment of any character or some new departure in 

 comparison with the parental types, and are they also 

 capable of surviving, propagating themselves and ulti- 

 mately becoming important elements of the population, 

 from which new races may in turn develop? Does 

 crossing ever lead to the appearance of new and pro- 

 gressive types by the disturbance it produces in the germ 

 plasm, rather than by the mere combination and redis- 



inal disturbance resulting from crossing? °The answers 

 to these questions are often very difficult, but upon these 

 answers depends in large part the status which will ulti- 



