A textbook of botany for colleges Volume 1-2

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 edition. Excerpt: ...thread of the fusion of the two sex cells, which brings together the parental chromosomes in equal contribution to the constitution of the offspring. It is only the mechanisms subsidiary thereto which vary. These mechanisms originate in a way to imply that the sexes were originally alike, and the differences between them arose through a division of labor, at first between the sex cells and later between the individuals which produce them, in connection with two subsidiary features of sexual reproduction,--viz. effecting the union of the sex cells, and nourishing (and later protecting) the embryonic offspring. Even in the highest plants and animals, sex seems to mean no more than this difference, developed to such a degree as to produce structures, organs, and even individuals, fitted to the respective parts taken by the sex cells. It is indeed possible that other factors are also involved in the result, but if so, they are obscure, while these are obvious. 8. Heredity, Variation, And Evolution The matters considered in the preceding sections lead naturally to others expressed in the title of this section. They are largely of theoretical, though very fundamental nature. Although in the past largely speculative in treatment they are now the subject of profound experimental researches, the conclusions of which apply equally to plants and animals. Heredity is the resemblance of an individual to its ancestors. Variation is the difference between an individual and its ancestors. It is easy to see how, granting the chromosome mechanism earlier described, heredity should occur. Indeed, on this basis, offspring should repeat their ancestors exactly, and the scheme leaves no room for variation at all. The student will note the phrase "like its...