222 Retrograde Varieties 



sive evolution. It is equally universal, per- 

 haps never failing. No great changes have 

 been attained, without acquiring new qualities 

 on one side, and reducing others to latency. 

 Everywhere such regressions may be seen. 

 The polypetalous genera Pyrola, Ledum, and 

 Monotropa among the sympetalous heaths, are 

 a remarkable instance of this. The whole evo- 

 lution of the monocotyledons from the lowest 

 > orders of dicotj^ledons implies the seeming 

 loss of cambial growth and many other quali- 

 ties. In the order of aroids, from the cala- 

 mus-root or sweet flag, with its small but com- 

 plete flowers, up to the almost wholly reduced 

 duckweeds (Lemna), almost an unbroken line 

 of intermediate steps may be traced showing 

 everywhere the concurrence of progressive and 

 regressive evolution, 

 ^c- ' Degressive evolution is not so common by far, 

 and is not so easy to recognize, but no doubt it 

 occurs very frequently. It is generally called 

 atavism, or better, systematic atavism, and the 

 clearest cases are those in which a quality which 

 is latent in the greater part of a family or 

 group, becomes manifest in one of its members. 

 Bracts in the inflorescence of crucifers are or- 

 dinarily wanting, but may be seen in some 

 genera, Erucastrum PolUchii being perhaps the 



