Systematic Atavism 647 



disposition may then be seen to extend to the 

 very center of the rosette, by use of microscop- 

 ical sections. Examining sections made in the 

 spring, the original arrangement of the leaves 

 of the stem is observed to continue until 

 the beginning of the growth of the shoot. 

 It is easy to estimate the number of leaves cor- 

 responding to a given number of spiral circuits 

 in these sections and the proportion is found to 

 indicate 13 leaves on 5 turns. These figures 

 are the same as those given above for the ordi- 

 nary arrangement of alternate leaves in the 

 main lines of the pedigree of the vegetable king- 

 dom. 



Leaving aside for the moment the subsequent 

 changes of this spiral arrangement, it becomes 

 at once clear that here we have a case of sys- 

 tematic atavism. The twisted teasels lose their 

 decussation, but in doing so the leaves are not 

 left in a disorderly dispersion, but a distinct new 

 arrangement takes its place, which is to be 

 assumed as the normal one for the ancestors 

 of the teasel family. The case is to be consid- 

 ered as one of atavism. Obviously no 

 other explanation is possible, than the sup- 

 position that the 5-13 spiral is still latent, 

 though not displayed by the teasels. But in 

 the very moment when the faculty of decussa- 

 tion disappears, it resumes its place, and be- 



