Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1920
Catalogue details

Licence: Public Domain Mark

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

    33/780 (page 9)
    use of them only can the great resemblance between the individuals grouped together in the same species be accounted for. When, as occasionally happens, members of different species are fertile inter se, the offspring is termed a hybrid, and hybrids in the majority of cases are not fertile. It has been pointed out, that whereas germs are in most cases Heredity exceedingly different from their parents, they never- and theless in process of growth come to resemble them. This tendency to reproduce the characters of the parent is called heredity. If the germ undergoes a large part of its development within a hard case, like a chick within the eggshell or in a cavity of the parent’s body, it is called an embryo; if it moves freely about, it is termed a larva. In the case of the development of an animal which has originated sexually, that is from the coalescence of two germs, the tendency is for it to assume characters intermediate between those of the two parents. Thus it is easy to see how sexual reproduction tends to annul the differences existing between members of the same species, by constantly producing means between them. When therefore a large number of individuals are found with very close resemblances, it is a reasonable supposition that the agent, which has caused this, is sexual reproduction; in other words, that they constitute a species. It is not however to be assumed that in every case conjugation results in the production of an animal exactly inter¬ mediate in character between the parents. In a large number of cases where father and mother differ from one another by some well-marked character unconnected with their sex the child resembles closely the father or the mother, a result denoted by the term prepotent applied to the parent which the offspring resembles. When however the children resulting from such a union are mated together, some of the grandchildren resemble one grandparent and some the other grandparent: three-fourths resemble the prepotent grandparent, and the character which they inherit is called the dominant character, and one fourth the other grandparent and the character which reappears in them is called the recessive character. When the grandparents differ in two characters, the same law holds with respect to each character, i.e. three-fourths of the offspring resemble one grandparent and one fourth the other— but not all the same children fall into the same group in the case of each character, and so some children inherit the dominant character from one parent and the recessive from another, and thus