xxiv] THE LAW OF AGE AND AREA 307 



representing Eu-callitriche, are abundant, whereas C. autumnalis 

 (Pseudo-callitriche) is rare and local. The two sub-genera of 

 Starworts are thus related to one another, as regards their dis- 

 tribution, in exactly the way that would be predicted, from their 

 degree of specialisation, either on the Differentiation Theory, 

 or on the Law of Age and Area. Further, it may be suggested 

 that the Duckweeds afford another case in point. Lemna minor, 

 which is the most widespread member of the family, also shows 

 indications of being the least specialised. These instances are 

 obviously too few for generalisation, but, as far as they go, they 

 show that the evidence from hydrophytes is decidedly favour- 

 able to the views of Willis and Guppy. It is greatly to be wished 

 that more test cases may come to light, in which certain species 

 within a genus, or genera within a family, can be accepted with 

 some degree of confidence as relatively primitive. 



