Experimental Pedigree-Cultures 565 



considered to constitute only retrograde steps, 

 and no advance. This conclusion has been fully 

 justified by some crossing experiments with 

 brevistylis, which wholly complies with Men- 

 del's law, and in one instance with nanella, 

 which behaves in the same manner, if crossed 

 with ruhrinervis. 



On the other hand, gigas and ruhrinervis, oh- 

 longa and alhida obviously bear the characters 

 of progressive elementary species. They are 

 not differentiated from lamarchiana by one or 

 two main features. They diverge from it in 

 nearly all organs, and in all in a definite though 

 small degree. They may be recognized as soon 

 as they have developed their first leaves and re- 

 main discernible throughout life. Their char- 

 acters refer chiefly to the foliage, but no less to 

 the stature, and even the seeds have peculiari- 

 ties. There can be little doubt but that all the 

 attributes of every new species are derived from 

 one principal change. But why this should af- 

 fect the foliage in one manner, the flowers in 

 another and the fruits in a third direction, re- 

 mains obscure. To gain ever so little an insight 

 into the nature of these changes, we may best 

 compare the differences of our evening-prim- 

 roses with those between the two hundred ele- 

 mentary species of Draba and other similar 

 instances. In doing so we find the same main 



