404 



THE 



TURALIST [Vol. XLIII 



determined as much by the modifications which the group 

 has previously undergone as by those to which the par- 

 ticular form has been subjected. 



I believe that these objections to the operation of selec- 

 tion in the evolution of these forms also argue against 

 De Vries 's mutation theory as an explanation, for, while 

 it is easily conceived that mutations may have arisen 

 within the limits of fluctuating variation in each case, 

 we must also assume that the new form pushed into the 

 new environment, or at least now occupies it to the ex- 

 clusion of immediately related forms, because better fitted 

 for it, which does not seem to be the case. What seems 

 to have actually taken place is that as each group pushed 

 out from the center of origin it became modified each 

 time it encountered a new region of environmental condi- 

 tions, not by the selection of forms better adapted to the 

 new conditions, but by the modification of the entire sec- 

 tion that invaded the new region. This may be tested 

 more thoroughly by an examination of the method of 

 evolution. 



Perhaps the most striking characteristic of this genus 

 is the manner in which evolution has taken place along 

 definite lines. Although the forms frequently have other 

 distinctive characters, they nearly all have this in common 

 that they are more dwarfed than the form from which 

 they have been derived, and there is no case in the genus 

 where a form is larger than its neighbor toward the center 

 . of origin. The history of each group has thus been one 

 of progressive dwarfing as it departed from the center 

 of dispersal. 



It is too often overlooked by students of evolution that 

 natural selection can cause directed evolution (orthoselec- 

 tion) ; for in order that it may do so it is only necessary 

 that there be an accruing advantage in the increased 

 development of a character. The characters must thus 

 be utilitarian, however, which is apparently not true in 

 this case. Moreover, it would certainly be taxing the 

 theory to make it account for the continued development 



