How Domestic Varieties Originate 223 



Those ambitious persons who are always looking for a 

 tuber-bearing tomato, therefore, might better concen- 

 trate their energies on the potato, for the tomato is not 

 developing in that direction; and even if the tomato 

 could be made to produce tubers, it would thereby lessen 

 its fruit production, for plants cannot maintain two diverse 

 and profitable crops at the same time. It is more rea- 

 sonable, and certainly more practicable, to grow potatoes 

 on potato plants and tomatoes on tomato plants. 



2. The quickest and most marked results are to be ex- 

 pected in those groups or species which are normally the 

 most variable. There are a greater number of variations 

 or starting-points in such species ; but it also follows that 

 the forms are less stable, the more the species is variable. 

 Yet the variations, being very plastic, yield themselves 

 readily to the wishes of the operator. Carri&re puts the 

 thought in this form: "The stability of forms, in any 

 group of plants, is, in general, in inverse ratio to the num- 

 ber of the species which it contains, and also to the degree 

 of its domestication." 



The most variable types are the most dominant ones 

 over the earth; that is, they occur in greater numbers 

 and under more diverse conditions than the compara- 

 tively invariable types do. The Compositse, or sunflower- 

 like plants, comprise a ninth or tenth of the total species 

 of flowering plants, and the larger part of the subordinate 

 types or genera contain many forms or species. Aster, 

 goldenrod, the hawkweeds, thistles, and other groups, are 

 representative of .the cosmopolitan or variable types of 

 composites. Whenever, for any reason, any type begins to 

 decline in variability, it usually begins to perish ; it is then 



