390 University of California Puhlications in Agricultural Sciences [Vol.2 



hybrids makes it probable that triploidy shouhl be accounted one 

 of the initial steps in the development of such Crepis species as C. 

 biennis and C. ciliata both possessing about 20 pairs of chromosomes. 

 As for the manner in which a polyploid race or a polyploid individual 

 may become an initiator of a new species, i.e., whether mutation, 

 hybridization, or both together may produce the specific differences 

 involved, that is still obscure. 



It may be considered as proved, however, that polyploidy in 

 Crepis causes profound changes in the biological properties of the 

 species involved. It should be emphasized here that a slowing down 

 of the rate of ontogenetic development occurs as a consequence of 

 polyploidy. It is perfectly clear that a delay of anthesis of only two 

 weeks may under certain conditions give a considerable advantage 

 to the plant. The rate of development may be affected even far more 

 by polyploidy, a circumstance which may play a decided role in the 

 invasion of an area by a new polyploid form, if this area were 

 formerly unavailable on account of climatic conditions. If the new 

 form is able to conquer a new territory, it will probably undergo 

 there the influence of a series of new conditions which were absent 

 in the original area. A result of such extension of the geographic 

 area occupied by a given species, which might be of the first impor- 

 tance in the evolution of a genus, would be the meeting and conse- 

 quent hybridization of species which have not previously been in con- 

 tact. If the new polyploid form remains in the original area, it will 

 nevertheless meet new opportunities of crossing with other later 

 blooming species, due to the delay of anthesis. The foregoing sug- 

 gestions apply to all weeds controlled by periods of grass cutting or 

 time of harvest as well as to other plants independent of the agri- 

 cultural activity of man. The importance, to agriculture, of the rate 

 of development of commercial plants is of course well known. 



It is of interest here to point out that the climatic frontier pre- 

 venting the further advance of a given form may itself become a 

 place of origin of polyploids due to peculiarities of temperature influ- 

 ences ; for it is very well known from experimental investigations that 

 unusual temperature conditions may be an important cause of the 

 production of polyploid gametes (de Mol, 1928; Belling, 1925), 

 After migration the polyploid forms thus produced on the limits of 

 distribution will be isolated, the climatic barrier preventing them 

 from meeting with the original form. The new additional polyploids 

 eventually formed along the limits of distribution might steadily 



