File:Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals (1907) (14769995881).jpg

Identifier: evolutionanimall00jord (find matches)
Title: Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals
Year: 1907 (1900s)
Authors: Jordan, David Starr, 1851-1931 Kellogg, Vernon L. (Vernon Lyman), 1867-1937
Subjects: Evolution
Publisher: New York, D. Appleton and Company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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her remotely ancestral or wholly new. From desirable vari-ations of this sort new races may be developed, each succeedinggeneration tending to give greater fixity. In general, wide crosses or hybrids are more successful withplants than with animals, because the mutual adjustment ARTIFICIAL SELECTION 89 traits become more important in the more highly specializedorganisms. Among animals, related species often cannot becrossed at all; the germ cells refuse to intermingle. Sometimesthere is a very imperfect mingling and the resultant animal isdivided within itself and does not live long. An example of thisis seen in Dr. Moenkhauss cross of the silverside (Menidia)with the killifish (Fundulus). The unmixed chromosomes of thegerm-cell nucleus are seen unblended, through several segmen-tations of the egg. In the case of the mule, the cross of the horse with the ass,the hybridization is readily effected, but the resultant offspringis sterile. Presumably the hereditary difference in the repro-
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FIG. 55.—Wild boar contrasted with modern domestic pig. (After Romanes.) ductive organs in the two parental strains is too great to allowthe normal development of generative organs in the progeny. In general, crosses between closely related species are fertile,the degree of fertility being less as the parent species are morewidely differentiated. Among animals, any great difference 90 EVOLUTION AND ANIMAL LIFE between the parent stocks renders hybridization impossible.But among plants, when hybrids are actually formed, fertilityrather than sterility may be taken as the rule. This is the casewith Mr. Luther Burbanks Primus berry, a cross between theSiberian raspberry (Rubus cratcegifolius) and the Californian dew-berry or blackberry (Rubus ursinus). In this form the fruitexcels in size and abundance either parent, and the hybridbreeds true from the seed, and ripens before either parent beginsto bloom. It was fixed in the first generation, being in this re-gard a rare exception to the

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