Again, a species of animal or plant is capable of being progressively improved by the skill of man, or, in other words, they can be made to assume, by improved culture and judicious selection, through successive generations, various modified forms and qualities which better enable them to minister to man's wants, than the species from which they were originally derived. Now, plants or animals, which culture or domestication have much altered from the normal condition of the species, are not alike in all particulars. Owing to some peculiarity of constitution, some are better adapted to one soil or climate than to another, and those suited to a given location arc found to possess various degrees of excellence. These, then, constitute the materials with which the cross-breeder or improver has to work; and it is highly desirable if not requisite, that he be well acquainted with his materials; he should know what has already been done, and be a good judge of plants or animals, as the case may be, in order that he may be better able to determine what remains to be accomplished, and what. varieties of flowers or fruits, or breeds of univer should study his subject point by point, put his thoughts on paper, and to this ideal standard of perfection, he should constantly aim.

This will save him from aiming at one thing at one time, and another thing another time - and there is little doubt that his stock would shortly assume a decided character.

Most important points to be attended to in selecting plants or animals, to breed from, are, that they shall be hardy, adapted to the climate, and free from disease. The certainty of produce, and consequently the profit to be derived from fruit trees in a given number of years, depends much on their hardiness, and on the power of their blossoms to withstand spring frosts; and there are many facts on record which seem to indicate that disease is hereditary in the vegetable, as it is in the animal kingdom.

The principal objects of cross-breeding are, to add vigor to the constitution; to modify, or get rid of defects, and to combine many good properties in one individual, in the shortest time. It is a common practice with the breeders of cattle, to obtain bulls now and then from other herds, rather than to breed from generation to generation, from their own stock - the object being, in some cases at least, to prevent the stock from becoming delicate - a cross from another herd being found to invigorate the constitution. And Mr. Knight, in his numerous experiments to obtain improved varieties of vegetables and fruit, "found that he obtained an increased vigor and luxuriance of growth when the fecundation of the blossoms of a variety was produced by the pollen of another kind."

Again, cross-breeding is resorted to with a view to obliterate defects, and to combine many excellencies in one individual, in the shortest time. Supposing, for instance, a man had a herd of Short-horns - good in all points, excepting that they were somewhat too . light in the hind quarters; he might, in the course of several generations, by culling his animals to breed from which showed this defect the least, so improve his stock as to bring it near to perfection; but, supposing instead of this, when made conscious of the defects of his herd, he at once obtained a Short-horn bull, good in all points, except that it was too full in the hind quarters, defective in the opposite degree; the result would probably be that the progeny would be more symmetrical than either of its parents, perhaps more so than if the bull had been perfect. Thus by one judicious cross, a breeder might cause his animals to attain a high degree of perfection in one generation, which, if he had confined himself to his own stock, would have taken him several generations to accomplish.

Until recent times - for it is but lately that the nature of the sexes of plants was clearly understood, (Mr. Knight being the first I believe to turn this knowledge to practical account,) men had to take advantage of any deviation which appeared in their crops naturally, or as the result of improved culture; and if the cultivator wished to perpetuate an improved variety, or obtain others better, he grew and seeded the plants alone; and if any seedling raised from it was better than the parent, that only was sown; seed from it sown; the best again selected, and so on through successive generations. A practice similar, it will be observed, to the breeding in-and-in of animals. Now, the practiced experimenter having a distinct object in view, a certain standard of excellence to aim at, carefully examines his plants, not with a view to select one but several; he notes their power to withstand adverse weather; their habit of growth; the character of their foliage; the abundance, form, substance, color, size and fragrance of their flowers; or the quantity, quality, size and beauty of their fruit, and the period it arrives at maturity.

If he finds a plant having one good point in perfection, and not remarkably defective otherwise, it is suited to his purpose; he may cross this with another plant having a second good point, and tolerably good in other respects, and may thus combine the two good properties in one had also marked defects, providing the defects of one were opposed to the defects of the other, so that if he could blend the two together a more perfect plant would be obtained than cither, these two might be suited to his purpose; he might cross them in the hope that the defects of one parent would counteract the defects of the other, and enable him to unite their two good properties in one individual, without the plant being otherwise objectionable. If, in the same season, two other plants, possessing two other good points, were crossed, and with a like result, the best of the seedlings obtained from each of these crosses, might be crossed, and it is probable that in the next generation a seedling would be raised in which the four good points would be combined. At the present day, however, the experimenter, instead of having to commence operations with an original species, has to deal chiefly with varieties already considerably improved.