The Italian poplar affords another instance. It is considered by some authors as a distinct species, _Populus italica_, and by others as a [624] broom-like variety of the _Populus nigra_, from which it is distinguished by its erect branches and other characters of minor importance. It is often called the pyramidal or fastigiate poplar. Its origin is absolutely unknown and it occurs only in the cultivated state. In Italy it seems to have been cultivated from the earliest historical times, but it was not introduced into other countries till the eighteenth century. In 1749 it was brought into France, and in 1758 into England, and to day it may be seen along roads throughout central Europe and in a large part of Asia. But the most curious fact is that it is only observed in staminate specimens; pistillate trees have not been found, although often sought for. This circumstance makes it very probable that the origin of the broom-like poplar was a sudden mutation, producing only one individual. This being staminate, it has been propagated exclusively by cuttings. It is to be admitted, however, that no material evidence is at hand to prove that it is not an original wild species, the pistillate form of which has been lost by vegetative multiplication. One form only of many dioecious plants is to be found in cultivation, as, for instance some South American species of _Ribes_. Total lack of historical evidence concerning [625] the origin of a variety has sometimes been considered as sufficient proof of a sudden origin. The best known instance is that of the renowned cactus-dahlia with its recurved instead of incurved ray-florets. It was introduced from Mexico into the Netherlands by Van den Berg of Jutphaas, under the following remarkable circumstances. In the autumn of 1872 one of his friends had sent him a small case, containing seeds, bulbs and roots from Mexico. From one of these roots a _Dahlia_ shoot developed. It was cultivated with great care and bloomed next year. It surprised all who saw it by the unexpected peculiarity of its large rich crimson flowers, the rays of which were reversed tubular. The margins of the narrow rays were curved backwards, showing the bright color of the upper surface. It was a very showy novelty, rapidly multiplied by cuttings, and was soon introduced into commerce. It has since been crossed with nearly all other available varieties of the _Dahlia_, giving a large and rich group of forms, bound together by the curious curling of the petals. It has never been observed to grow in Mexico, either wild or in gardens, and thus the introduced individual has come to be considered as the first of its race.