44 



Mutability and Individual Variation. 



this term. For they stand in absokitely different relation 

 to our thesis. 



The following groups of phenomena usually fall 

 within the meaning of the term variability: 



Fig. 8. Hedera Helix var, arhorea} 



1. Systematic polymorphism and its supposed causes. 



2. Polymorphism caused by crossing. 



^ The best known example is afforded by Hedera Helix arhorca 

 whicli is offered by many nurseiymen as var. arborca. It is not a 

 variety, but consists simply of the erect flowering shoots cut off the 

 ordinary ivy, stuck in the ground, and grown as trees. In April i88S 

 I made some such cuttings, and have cultivated the best one till the 

 present time. It forms a richly branched bush over a meter higli 

 (Fig. 8). As is shown in the figure at a, b, c, creeping branches 

 arise from time to time. In 1893 I sowed the berries of an older 

 plant of this kind, in this case an ivy bush of about two meters, and 

 obtained over a thousand seedlings. These still grow in our garden 

 and have made, up till now, exclusively creeping stems and branches. 

 The Arborea-iorm is evidently not inherited. Similar phenomena 

 occur in many other genera, for example in the creeping species of 

 figs in South Europe ; but they have not been sufficiently investigated. 



