xii INTRODUCTION 



insects are not, we must assume, to be credited with llie power of discrimination 

 between Ibrms differing in other respects, wiiere those features do not all'ect their 

 search for the above items of food. 



The relative scarcity of impure crosses or hybrids shows how unerring is the 

 instinct of insects. Hybrids, as distinct from species and varieties, are not the 

 main links in the chain of evolution of species. They are exceptional illustrations 

 of the influence of cross-pollination in the production of new forms. Incidentally 

 they may be the origin of new species, but they illustrate the manner in which 

 species or varieties may occur, since they are the results of the same process which 

 brings about evolution of forms in one direction, by cross-pollination. Mutations, 

 &c., produced in self-fertile plants are illustrations of a similar process of evolution 

 brought about by other means, and due presumably to the inherent tendency to 

 vary (where variation occurs), which is obscured somewhat in crossed plants by the 

 influence of the different characters of the parents in a cross-pollinated plant, or, at 

 any rate, the possibility of their difference, since distance may in such cases be 

 responsible for variation in the two individuals crossed, whereas in self-pollination 

 no such difference exists, and the plant is grown under similar conditions. 



At one time it was considered hybrids were rare, and generally sterile. A 

 Frenchman, Naudin, disproved this, and found many hybrids capable of fertili- 

 zation. 



It was reserved for the Austrian monk, Gregor Mendel, to explain and express 

 arithmetically the principles of hybridity. He found the result of crossing two 

 different forms was not the production of intermediates, but of plants with the 

 characters of one of the parents. Wrinkled and smooth-seeded peas when crossed 

 produced hybrids with round peas in the first generation, and he termed the round- 

 pea type a dominant, whilst the other type was recessive. The hybrid, when 

 pollinated with its own pollen, in the second or Fo generation, produced some 

 plants with round, some with wrinkled peas, in the proportion of 3 to i; i.e. a 

 quarter of the descendants of a generation revert back to the original with a 

 recessive character. The actual result in the first generation may be of an inter- 

 mediate type, and in the F., generation some of both characters of the parents, 

 and some intermediates, will occur. The resultant hybrid may appear to be inter- 

 mediate, when really the result is a combination of two dominant characters giving 

 the semblance only of an intermediate. These principles can be readily worked out 

 where the parent plants do not present a complex of characters each of which is 

 likely to influence the resulting hybrid. In such cases the elimination of each 

 character separately is the only means of determining the nature of the parents in 

 a first cross, and it becomes more difficult in a second or third cross. 



Amongst British wild plants some genera are more liable to hybridize than others. 

 Exactly which are species and which are hybrids, indeed, is not certainly known. 

 Willow hybrids are generally fertile, and have been universally accepted as such. 

 Many so-called species of Mentha are sterile hybrids, and are largely reproduced 

 by suckers. It is also not clear whether some genera do not include some hybrids 

 that are usually sterile and others that are fertile. Formerly hybrid varieties were 

 held to be fertile, hybrid species sterile, but this view needs some modification 

 to-day. 



Amongst British genera hybrids occur, e.g. in Raiiuiiailus, Papaver, Cardamine, 

 Helianthenium, Viola, Silene, Lychnis, Hypericum, Medicago, Rubtis, Geinn, Rosa, 

 Drosera, Epitobiuni, Galium, Erigeron, Senecio, Carduiis, Hieracium, Vaccinittm, 

 Erica, Limonium, Primula, Gentiana, Piilinonaria, Verbascuni, Linaria, Euphrasia, 

 Mentha, Lainiiuii, Polygonum, Rumex, Daphne, lietula, Ulmus, Ouercus, Sali.x, 



