Chap. XVII. EVIL FROM INTERBREEDING. 127 



Plants. 



When a single plant of a new species is introduced into any country, 

 if propagated by seed, many individuals will soon be raised, so that 

 if the proper insects be present there will be crossing. With newly- 

 introduced trees or other plants not propagated by seed we are not here 

 concerned. With old-established plants it is an almost universal practice 

 occasionally to make exchanges of seed, by which means individuals 

 which have been exposed to different conditions of life,— and this, as we 

 have seen, diminishes the evil from close interbreeding,— will occasionally 

 be introduced into each district. 



Experiments have not been tried on the effects of fertilising flowers with 

 their own pollen during several generations. But we shall presently see 

 that certain plants, either normally or abnormally, are more or less sterile, 

 even in the first generation, when fertilised by their own pollen. Although 

 nothing is directly known on the evil effects of long-continued close inter- 

 breeding with plants, the converse proposition that great good is derived 

 from crossing is well established. 



With respect to the crossing of individuals belonging to the same sub- 

 variety, Gartner, whose accuracy and experience exceeded that of all other 

 lrybridisers, states 40 that he has many times observed good effects from 

 this step, especially with exotic genera, of which the fertility is somewhat 

 impaired, such as Passiflora, Lobelia, and Fuchsia. Herbert also says, 41 

 " I am inclined to think that I have derived advantage from impreg- 

 " nating the flower from which I wished to obtain seed with pollen from 

 M another individual of the same variety, or at least from another flower, 

 " rather than with its own." Again, Professor Lecoq asserts that he has 

 ascertained that crossed offspring are more vigorous and robust than their 

 parents. 42 



General statements of this kind, however, can seldom be fully trusted ; 

 consequently I have begun a series of experiments, which, if they continue 

 to give the same results as hitherto, will for ever settle the question of the 

 good effects of crossing two distinct plants of the same variety, and of the 

 evil effects of self-fertilisation. A clear light will thus also be thrown on 

 the fact that flowers are invariably constructed so as to permit, or favour, 

 or necessitate the union of two individuals. We shall clearly understand 

 why monoecious and dioecious,— why dimorphic and trimorphic plants exist, 

 and many other such cases. The plan which I have followed in my expe- 

 riments is to grow plants in the same pot, or in pots of the same size, or 

 close together in the open ground; to carefully exclude insects; and then 

 to fertilise some of the flowers with pollen from the same flower, and 

 others on the same plant with pollen from a distinct but adjoining plant. 

 In many, but not all, of these experiments, the crossed plants yielded much 

 more seed than the self-fertilised plants; and I have never seen the 



40 ' Beitrage zur Kenntniss der Befruchtung,' 1844, s. 366. 



41 ' Amaryllidaceee,' p. 371. 



42 



' De la Fecondation,' 2nd edit., 1862, p. 79. 



