Systematic Atavism 637 



it may be seen to occur as an anomaly, incom- 

 pletely developed, often very rare and with all 

 the appearance of an accidental variation, but 

 sometimes so common as to seem nearly normal. 



Coming now to particular instances, we may 

 turn our attention in the first place to the genus 

 Sisymbrium. This is a group of about 50 species, 

 of wide geographic distribution, among which 

 the hedge mustard {S. officinalis) is perhaps the 

 most common of weeds. Two species are re- 

 puted to have bracts. Sisymbrium hirsutum and 

 S. supinum. Each flower-stalk of their long 

 racemes is situated in the axil of such a bract, 

 and the peculiarity is quite a natural one, corre- 

 sponding exactly to what is seen in the inflor- 

 escence of other families. Besides the Sisym- 

 brium some six other genera afford similar 

 structures. 



Erucastrum PolUchii has been already allud- 

 ed to in a former lecture when dealing with the 

 same problem from another point of view. As 

 previously stated, it is one of the most manifest 

 and most easily accessible examples of a latent 

 character becoming active through systematic 

 atavism. In fact, its bracts are found so often 

 as to be considered by some authors as of quite 

 normal occurrence. Contrasted with those of 

 the above mentioned species of Sisymbrium, 

 they are not seen at the base of all the flower- 



