OS NATURAL HYBRIDS. 211 



each other, but which, when growing together, are (hybridity 

 excluded) not unfrequently accompanied by forms externally inter- 

 mediate, and, mirabile dictu, presenting the entirely new character 

 of total sterility ! Secondly : we do not know of any variable species 

 in which the two extremes are perfectly fertile, while the inter- 

 mediates are entirely sterile, all being under the same environment. 

 But the phenomena described exactly agree with what we know to 

 be the facts resulting from cross-fertilisation, and they are also 

 exactly represented among wild plants by such hybrids as Viola 

 canina x liiviniana, for example. Until, then, our opponents" can 

 explain these " sterile intermediates' 1 in some other way, we are 

 justified in assuming them to be what they exactly resemble, 

 without testing the matter by artificial crossing in every case.* 

 Speaking of past and present glaciation, Dr. Wallace writes in 

 Island Life (ed. 2, p. 108) : — " The two sets of phenomena are so 

 exactly identical that no one who has ever compared them can 

 doubt that they are due to the same causes," These words apply 

 admirably to the case as it stands between natural and artificial 

 hybrids ; but we can go further, for many wild hybrids have had 

 their counterparts produced by artificial crossing in gardens. 



When an intermediate form is fertile, or nearly so, it is naturally 

 a more difficult question to decide whether it be an intermediate 

 form connecting two supposed distinct species, or whether it be 

 really a fertile hybrid. 



In conclusion, I append a list containing a small selection of 

 hybrids from Dr. Focke's Pfianzen-Mischlinge ; the parents of these 

 are natives of Britain, but the hybrids have most of them not yet, 

 I believe, been noted in this country. I have omitted all allusion 

 to various critical genera which are already the subject of special 

 study here, as well as many other forms, for one reason or another : — 



Banunculus acris x bulbosus ; R. Sardous x sceleratus, 



Papaver dubium x somniferum"; P. Bhceas x somniferum ; P. 

 dubium x Rhoeas. ~ 



Nasturtium amphibium x silvestre ; N. palustre x silvestre; N. 

 palustre x amphibium. 



Cardamine amara x silvatica. 



Helianthemum Chamaecistus x polifolium. 



Viola arenaria x canina; V. arenaria x u silvatica"; V. lactea 

 x Rfviniana. 



Dianthus deltoides x Armeria. 

 Lychnis diurna x alba. 

 Stellaria graminea x nliginosa. 



Hypericum perforatum x dubium; H. perforatum x tetr apt erum; 

 H. dubium x tetrapterum ; H. humifusum x perforatum. 

 Medicago falcata x sativa. 



Potentilla argentea x Tormentilla. 

 Alchemilla alpina x vulgaris. 

 fyrus communis x Aria; P. Mai 

 Aria ; P. Aria x torminalis ; P. Aucui 



* For some facts as to sterile hybrid Violets, see Juurn. But. 1889, p. 226. 



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