Page:The New International Encyclopædia 1st ed. v. 10.djvu/414

* HYBBIDITY. 358 HYBBIDITY. sterility of hybrids. Wallace has a somewhat different view. A new speties arises in con- nection with some slightly varjinj; conditinns o! ciiviroiiiucnt. If these are sullicient to create a chan^ in coloratiim or form, they will, by virtue of correlation. atVect also the perm cells, and may give rise to a certain amount of in- fertility. Now the infertility will be beneficial whenever new species arise in the same area with the parent form, because it would result in a destruction of both of the adapted forms. Con- sequently, whenever two species which are at the same time adapted and infertile arise, they will retain their respective adapted conditions and will survive. Finally, t'atclipool ami Romanes have independently suggested an explanation that is worthy of consideration — it is tli:it the sterility of species is not something acquired, but is primary. Whenever from any cause two lots of individuals of one species become partly in- fertile rn^rr se, those lots are, as it were, seg- regated. Each of them can develop its own way and give rise to a distinct species. If the species were not tlius sterile at the beginning, differcn- tiation would be difficult. (2) Kriation to Inheritance. — ^Hybridity is im- portant for the study of the laws of inheritance. Since the parents are more unlike, the nature of their combinations is still more interesting than in the case of ordinary se.xual reproduction. Hybrids are particularly apt to show a reversion to an ancestral condition. esi*cially when op- posing characters are intermingled. The follow- ing rules concerning hybrids have been formulat- ed by Kocke: (a) ''All the individuals formed by the crossing of two pure species ur races are, if they have been produced and grown under the fame conditions, exactly like each other as a rule, or they differ hardly more than specimens of one and the same species are apt to do." This propo- sition is subject to many exceptions. In certain crosses the female clement is predominant in the progeny; in others the male. Especially in cases of alternative heritage (see Herkoity) the hybrids may Ix; of two types, the one resem- bling more the one parent, the other type the other parent. (b) "Tlie characteristics of the two crosses may be different from those of the parent species. They differ most from both parent species in size and luxuriance, as well as in fertility." In the ca.se of alternative heritage an interesting rule was worked out by Mendel. He finds that one of the species is prepotent (p), the other subpotential (s) . The hybrids will be either of the ;) or the .f species, and the relative proportion will be p 75 per cent., s "25 per cent. If. now, self-fertilization occur, 50 per cent, of me plants will produce either p or s progeny in the proportions 75 per cent, p, 25 per cent. s. All the descendants of p and s breed true. The consequence of Mendel's 'law of dichotomy' in hybrids is that the proportion of the pure races is constantly increasing in the successive genera- tions descended from a hybrid. "Malformations and curious forms are much more common, espe- cially in the flower parts of hybrids, than in in- dividuals of a pure descent." Double flowers ap- pear to be formed especially easily in hybrids. Hybrids are frequently infertile, because the pol- len is imperfectly fonued. (3) Utility. — Hybridity has great practical utility in agriculture. Many varieties of our domesticated plants have arisen from artificial hybrids. The first generation of hybrids usually gives few results, but great variations continue lo appear in subsequent genenitiuns. Thus, when two species of hybrid toliaeeous pbiiits are crossed, the leaves of the second or third genera- tion arc often highly variable. This great varia- bility, on the other hand, renders any favorable variation liable to be lost unless great pains are taken to fix the desired quality by inandin breeding, or unless the quality can be reproduced vcget.itively — i.e. by cuttings and grafts. The diminished fertility of some hybrids is agricul- turally advant.-ige<]us in the case of plants that can lie reproduced vegetatively. .Some of the .^dvantageous results of hybridization are as fol- lows: (a) Increased Size and Vigor. The hybrid between the English walnut and the California black walnut is of exceptional value, "for it grows twice as fast as the combined growth of both parents. The woo<l is very compact, with lustrous silky grain," etc. Another hybrid of the black walnut and the California bl.ack walnut produces fruit of much larger size than that of either (^arent. These are illustrations of a com- mon result of hybridization — increased size and vigor, (b) Increased Hardiness. The pro<luction of hardy r.ices of plants is very importnnt, be- cause it carries farther north the limits of agri- culture and increases the area of cultivation. Every .few years the orange and lemon trees of the Gulf Stales and California are greatly dam- aged by freezing weather. The Japanese trifoli- ate orange is hardy, but has fruit of small size. It is hoped that a hybrid may be obtained be- tween it and the ordinary orange which shall have increased hardiness without loss of size or flavor in the fruit, (c) Adaptation to Warmer t^'limate. The Kieffer and T>>Conte pears, which have revolutionized pear agriculture by extend- ing the range of profitable pear-growing hundreds of miles southward, are said to be hybrids be- tween the common pear and the Chinese sand pear, (d) Increased Kcsistance to Disease. When the vast grape industry of France was thrcatene. While the percentaire of sugar in the sugar-beet has been hitherto chiefly increased by selection, there seems to be a chance for improvement by hybridization. A hybrid cinchonii produces three to four times the ordi- r.ary percentage of quinine, (f) Change of Sea- son and Duration of Life. By hybridization plants have arisen fruiting at unusual times, like Bur- bank's blackberry and raspberry hybrids. A fair- ly successful attempt to make the pansy peren- nial resulted from crossing the garden pansy with the perennial Viola cornuta. (g) .Acquisi- tion of Odor. The frairrance of the pansy has been improved by crossing with odoriferous Al- liine species, (h) Improvement of Quality and Flavor. This result is illustrated by the case of smoking tobaccos, in which the superior-flavored Havana race has been crossed ^vith other races having larger leaves, so that a greater quality of well-flavored leaf has been produced. Alto-