No. 564] THE FIXATION OF CHARACTER 715 



not random and entirely unpredictable ones, for we are 

 able to distinguish certain definite parts of the plant and 

 animal body which throughout larger or smaller groups 

 of organisms are characteristic seats of conservatism, 

 and others which are everywhere subject to continual 

 change. The urinogenital, nervous and skeletal systems 

 of vertebrates, and to a certain extent of invertebrates 

 as well, are typically conservative and subject to com- 

 paratively slight alteration during evolutionary develop- 

 ment. Certain definite regions of the body, such as the 

 skeleton of the mammalian neck, are more definitely 

 stereotyped than others as to the number and arrange- 

 ment of parts. The extreme conservatism of the repro- 

 ductive organs of all plants has of course long been recog- 

 nized and has been proven by a study of internal as well 

 as of externa] structure. More recently it has been demon- 

 strated, that the woody axis, as well, is the seat of firmly 

 fixed and therefore ancient characters. Each main divi- 

 sion of the vascular plants has a fundamental stelar plan, 

 and every subordinate group has its peculiar and specific 

 type of wood structure which is exceedingly constant in 

 individuals otherwise very different and, as a diagnostic 

 character for families and sometimes smaller groups, is 

 therefore of much value. The axis of the root is especially 

 conservative and has remained practically unchanged in 

 its general plan throughout the entire evolution of woody 

 plants. The vascular svstem of the leaf, especially at 

 the node where the leaf and stem unite, has many times 

 been found to display primitive features wholly lost 

 elsewhere. In such conservative systems and regions it 

 is not all the characters which have become constant, but 

 only those which we have called typically conservative, 



particular stages, notably the earlier ones, in the life 

 history of the individual are much less subject than the 



