The most curious and most suggestive instance of such an alternation is the case of the water-persicaria or _Polygonum amphibium_. It is known to occur in two forms, one aquatic and the other terrestrial. These are recorded in systematic works as varieties, and are described under the names of _P. amphibium_ var. _natans_ Moench, and _P. amphibium_ var. _terrestre_ Leers or _P. amphibium_ var. _terrestris_ Moench. Such authorities as Koch in his German flora, and Grenier and Godron in their French flora agree in the conception of the two forms as varieties. Notwithstanding this, the two varieties may often be observed to sport into one another. They are only branches of the same plant, grown under different conditions. The aquatic form has floating or submerged stems with oblong or elliptic leaves, which are glabrous and have long petioles. The terrestrial plants are erect, nearly simple, more or less hispid throughout, with lanceolate leaves and short petioles, often nearly sessile. The aquatic form flowers regularly, producing its peduncle at right angles from the floating stems, but the terrestrial specimens are ordinarily seen without flower-spikes, which are but rarely met with, at least as far as my own experience goes. Intermediate [434] forms are very rare, perhaps wholly wanting, though in swamps the terrestrial plants may often vary widely in the direction of the floating type. That both types sport into each other has long been recognized in field-observations, and has been the ground for the specific name of _amphibium_, though in this respect herbarium material seems usually to be scant. The matter has recently been subjected to critical and experimental studies by the Belgian botanist Massart, who has shown that by transplanting the forms into the alternate conditions, the change may always be brought about artificially. If floating plants are established on the shore they make ascending hairy stems, and if the terrestrial shoots are submerged, their buds grow into long and slack, aquatic stems. Even in such experiments, intermediates are rare, both types agreeing completely with the corresponding models in the wild state. Among all the previously described cases of horticultural plants and monstrosities there is no clearer case of an ever-sporting variety than this one of the water-persicaria. The var. _terrestris_ sports into the var. _natans_, and as often as the changing life conditions may require it. It is-true that ordinary sports occur without our discerning the cause and without [435] any relation to adaptation. This however is partly due to our lack of knowledge, and partly to the general rule that in nature only such sports as are useful are spared by natural selection, and what is useful we ordinarily term adaptive.