'54 THE BREEDING OP PLANTS 



choose those that are already adapted, as it is also his part to determine what kinds of fertilizers he 

 shall use or what kinds of crops he shall grow. Good farmers have always been plant-breeders : they 

 have "selected the best" for seed ; they have changed seed from place io place ; they have exercised 

 a shrewd discrimination in varieties and strains. The present phase of plant-breeding differs in attach- 

 ing more importance to plant adaptations and in a better understanding of the principles underlying the 

 practices. The good stockman does not use common stock for breed6rs ; the good plantsman does not use 

 common stock for breeders. , 



Every good farmer, then, is of necessity a plant-breeder. He knows the points and merits of his 

 wheat or cotton, as the dog-fancier knows the points of his dogs. Knowing this, he will also know what 

 improvements are needed to adapt the plants to his soil or climate or system of farming or markets. 

 He will then set about it to secure these improvements by (1) looking for plants that most nearly 

 approach the ideal or causing them to vary toward that ideal, (2) selecting seed from these plants, 

 (3) repeating the process as long as he lives. The remainder of the work is detail. 



This process may not produce any very striking or permanent new vegetable forms ; but the 

 efficiency of a personal business lies mostly in these smaller grades of differences. If a man is a seller of 

 new plants, he may want plants with new names. For certain regions and certain purposes, also, wholly 

 new kinds of things may be needed ; but with the producing of these the individual farmer will not 

 often concern himself. It is significant that some of the most important seed business of the present day 

 rests on the sale of improved, selected or pedigreed seed of standard varieties. Every ambitious, careful 

 and clear-headed farmer should now be able to produce superior seed-stock of his staple crop to sell for 

 planting at good living prices. The public is now ready to believe that there are grades of quality in 

 seed-stock of the common crops as there is in butter or cheese or liquors (some time we will also know 

 that there are grades of quality in plain drinking-water). 



The above advice rests on the principle that improvement is made by means of selection. This is the 

 Darwinian principle. Selection, however, rests on variation. Why variations (or differences) arise, 

 nobody really knows, although nearly everybody has an opinion. It is known, however, that variations 

 accompany changes in soil, climate, methods of growing, and other changed conditions. Variation may 

 also be induced or started off by crossing one plant with another, and such differences are likely to be 

 marked. Some variations appear without any apparent reason, and they may be more or less stable from 

 the first; they are "sports," or, as we now say, mutations (following the terminology of DeVries). 

 These marked so-called "sudden" variations may reproduce remarkably true from seed. The recent 

 evolution discussions have tended to divide variations into these two classes, — the small individual 

 variations that do not reproduce or " come true " (and are therefore presumed to be of no permanent 

 effect in the evolution of the type), and the variations, usually wider, that do '' come true." We do not 

 know, however, what are the ultimate origins or what the physiological differences. Divested of technical 

 questions and controversial phases, the practical difference between mutations and other variations is one 

 of definition, — the mutations come true, the others do not. The mutation theory controverts the older 

 doctrine that variations may be augmented by selection until the differences become morphologically 

 great, and until they also become "fixed" or able to reproduce themselves, — that is, that species originate 

 by means of selection ; but the mutation theory does not controvert the importance — but rather empha- 

 sizes it — of selection as an agent in the improvement of agricultural plants. Even if a mutation (or 

 hereditable variation) appears, it may still be greatly improved in its minor features by careful selection. 



The mathematical law of chance or probabilities applies to hybrids as well' as to other numerical com- 

 binations. If a plant with three given characters, for example, were to be crossed with a plant of three 

 contrasting characters, the law of probability would predict about how many of the offspring would 

 have one combination of characters and how many would have another combination. The law might not 

 be exemplified in any one plant, but it would very likely be apparent in the average of a number of 

 plants ; and the greater the number, the more regular the results, due to the subordination of exceptions. 

 Mendel found that this law applies to characters that are united in crossing ; if the law applies, it 

 means that the characters or marks have an identity or individuality of their own, that they are carried 

 over entire rather than as blends. In order to explain the application of the mathematical law of 

 chance to hybridization, therefore, we suppose that characters are units and that they are represented 

 directly in the germ-cell ; and hereby arises the theory of the " purity of the germ-cell." That is to say, 

 the mathematical law requires a biological hypothesis to explain why or how it works with animals and 

 plants. Very many experiments have shown that the characters of parents reappear in offspring approxi- 

 mately in the given mathematical proportions ; on the other hand, other experiments show a different or 



