344 Ever-sporting Varieties . 



Pinnate and palmate leaves are no doubt 

 derivative types. They must have originated 

 from the ordinary simple leaf. The monophyl- 

 ly may therefore be considered as a reversion to 

 a more primitive state and the monophyllous 

 varieties may be called atavistic. 



On the other hand we have seen that these 

 atavistic varieties may revert to their nearest 

 progenitors, and this leads to the curious con- 

 ception of positive and negative atavism. For 

 if the change of compound leaves into single 

 ones is a retrograde or negative step, the con- 

 version of single or ternate leaves into pinnate 

 and palmate ones must evidently be considered 

 in this case as positive atavism. 



This discussion seems to throw some light on 

 the increase of leaflets in the clover. The pea- 

 family, or the group of papilionaceous plants, 

 has pinnate leaves ordinarily, which, according 

 to our premises, must be considered as a deriva- 

 tive type. In the clovers and their allies this 

 type reverts halfway to the single form, pro- 

 ducing only three leaflets on each stalk. If now 

 the clover increases its number of leaflets, this 

 may be considered as a reversion to its nearest 

 progenitors, the papilionaceous plants, with pin- 

 nate leaves. Hence a halfway returning and 

 therefore positive atavism. And as I have al- 

 ready mentioned in a former lecture, evenly 



