NATURAL HYBRIDS. 585 



About two hundred hybrids, for the most part fertile, have come into existence 

 in nature within the family of the Rosaceae. The genera Geum, Potentilla, Rubus, 

 Rosa, and Sorbus are inexhaustible in the formation of hybrids. On the other 

 hand, the Papilionaceae, a family allied to the Rosaceae, is peculiarly wanting in 

 hybrids. 



This discrepancy between nearly-related families in respect of the number of 

 their hybrids is again noticeable in the class of plants known as the Monopetalse. 

 The Labiatae, e.g. the genera Ajuga, Prunella, Calamintha, Lamium, Marrubium, 

 Mentha, Salvia, and Stachys include numbers of hybrids, whilst the Boraginaceae 

 have very few. Of the latter only one or two hybrids are known, and these belong 

 to the genera Pulmonaria and Symphytum. The Scrophulariaceae and Rhinanthaceae 

 have long been noted for the great variety of their hybrids; the genera Pedicularis 

 and Verbascum in particular exhibit a great wealth of forms, Pedicularis in alpine 

 regions and Verbascum on the shores of the Mediterranean. Verbascum alone 

 includes more than fifty. In the case of Gentianaceae, also, the products of a great 

 number of different crosses are found in the Alps, the chief parent-species being the 

 long-stalked Gentiana lutea, G. Pannonica, G. punctata, and G. purpurea. Pri- 

 mulaceae, whether growing on the upper or the lower levels of mountains, are 

 famous for the abundance of their hybrids. In the genera Androsace, Primula, 

 and Soldanella the number of hybrids identified by Botanists surpasses the number 

 of the species from which they have sprung. The Ericaceae, although comparatively 

 poorly represented in Europe, exhibit several hybrids derived from the genera Erica, 

 Rhododendron, and Vaccinium. 



Hybrids are also specially numerous in the family of the Rubiaceae, the genus 

 Galium being the most prolific in this respect. But the greatest number of all is 

 to be found amongst the Compositae. More than two hundred hybrids have been 

 identified in the genera Achillea, Carduus, Centaurea, Cirsium, Hieracium, Inula, 

 and Lappa alone. Of hybrid Compositae the following are particularly worthy of 

 notice: Erigeron Hulsenii, which is the result of a cross between Erigeron 

 Canadense, an annual species brought into Europe from America, and Erigeron 

 acer, a perennial species indigenous to Europe, and, secondly, the Cudweed hybrids, 

 .Filago mixta, F. neglecta, F. subspicata, &c., which spring from crosses between 

 .annual species, and are, therefore, exceptional, for annual species in general rarely 

 produce hybrids. 



In many cases only a few individual instances of these natural hybrids have 

 been observed; in some, indeed, one single specimen alone has as yet been discovered. 

 'The majority, however, grow by the hundred or by the thousand in the localities 

 proper to them, and many are represented by immense numbers of plants, and 

 possess a wide area of distribution. Salvia betonicifolia, a hybrid derived from 

 .Salvia nemorosa and S. nutans, occurs as commonly as its progenitors in places on 

 the grassland in the central parts of Siebenbiirgen (Hungary); Marrubium remotum, 

 -a hybrid of Marrubium peregrinum and M. vulgare, is to be found everywhere on 

 -the plains of South-Eastern Europe, especially in the flat country by the Theiss and 



