176 Changing Plant and Animal Nature 



of life and they have taught us a great deal about the nature 

 of living things. But they have in some respects evaded our 

 question regarding species. When a seedless orange appears 

 upon a respectable orange tree it is necessary merely to pre- 

 serve the twig or branch that bore the freak, and to en- 

 courage it to bear more. It is of course impossible to plant 

 seeds for the purpose of raising more seedless orange trees. 

 It is possible, however, to make grafts and cuttings. With 

 many species of plants it is possible to get more individuals 

 by having twigs or branches continue their growth after 

 being detached from the parent stock. Similarly it is possi- 

 ble to propagate many plant varieties by means of bulbs, 

 tubers, root stocks and other vegetative parts. Thus we can 

 preserve the qualities we desire in a constant succession of 

 new individuals. The fact that these plants can be propa- 

 gated by vegetative means is relied upon for the practical 

 purpose of getting more of the desired kinds; but it leaves 

 us in the lurch if we are intent upon answering the question 

 whether one of these deviates is a " true species," for it avoids 

 the test of consistent reproduction. 



There are other plants that reproduce only by seeds — 

 that is, by a true sexual process. From these we can tell 

 whether the qualities that concern us, the new qualities that 

 characterize the strain, enter into germinal continuity. We 

 may consider the hundreds of varieties of wheat and corn 

 and other cereals, of beans and peas and vetches and lentils, 

 of cotton and tomatoes, and of annuals grown for their flow- 

 ers. These varieties or strains breed true, yielding constantly 

 new individuals with the characteristics for which they were 

 selected in the first place. 



In many cases these new strains are the result of hy- 

 bridizing or crossing of two strains. This is only to say, how- 

 ever, that 



( 1 ) there are varieties to be crossed ; - ^ 



(2) new strains can be produced by crossing; and 



( 3 ) hybridizing may result in the production of stable 

 and self-sustaining varieties. . 



