1. The keeled form of Paludestrina jenkinsi is described and its distribution discussed.
2. The keel is found to be non-heritable through three parthenogenetic generations.
3. The evidence for the (a) environmental and (b) autogenous origin of the keel is analysed. The result is a presumptive case in favour of the former, though such a keel is not among the structural modifications which have been proved, either by experiment or otherwise, to be produced by environmental factors.
4. The various modifications of the keel show that, if of environmental origin, a more complex structural system is produced than is usual in such cases.
5. It is suggested that, as the keel (or one of its derivatives) is found as a fixed character in another species of the same genus and in allied genera, it is deprived at present of the means of attaining genetic stability in P. jenkinsi.
- Copyright © 1926 The Company of Biologists Ltd.