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Part III. The Fertility Op The Parent Species And Hybrids.
GENETIC STUDIES ON A CAVY SPECIES CROSS.
The genetic studies herewith presented were made possible for the author, by the reception of the foundation stock, in December 1909, from Dr. W. E. Castle. The first crosses had been made in 1903, and about 200 of the wild and intense wild-blooded hybrid animals had been born when the stock was received. The birth records, the weights, and such skeletons as had been saved, as well as the living hybrids, were made available to the author, who here expresses his gratitude for the privilege of using this material and for generous assistance, which was never withheld. He also wishes to acknowledge the valuable aid of Mr. Elmer Roberts, in the preparation of the manuscript.
Most of the manuscript was written and most of the data were analyzed at the College of Agriculture of the University of Illinois, to which the author is deeply indebted for liberal use of time and facilities.
I. THE SYSTEMATIC POSITION OF THE PARENT RACES.
This paper is based on a study of the wild Brazilian guinea-pig, (Cavia rufescens Lund), the common domestic guinea-pig (Cavia porcellus Linn.), hybrids between these, and subsequent progeny obtained in the next eight generations by various matings. About 1,800 animals, wild or hybrid, enter in one way or another into experiments on color, growth, size, and fertility. Besides these, approximately 600 guineapigs, living under the same conditions in collateral experiments, serve as a basis for necessary comparisons.
That the hybrids are the result of a species cross rather than a variety cross can hardly be doubted, since the § wild and j wild males are entirely sterile. In order to meet any doubt or criticism at the outset, I may briefly give my reasons for assigning the parent stocks to such diverse and distantly related species. In the summer of 1903 Dr. W. E. Castle received one wild male and two wild females from Mr. Adolph Hempel, Campinas, Sao Paulo, Brazil. These and their progeny were kept for some time at the Harvard Zoological Laboratory, and were removed later to the Laboratory of Genetics, Bussey Institution, Harvard University. In the summer of 1911, three years after the last animal of pure wild pedigree had died, we again received from Mr. Hempel one wild male and one wild female. At first it was thought that these wild cavies belonged to the commonly described Cavia aperea Erxleben, but a more careful investigation showed later that they belonged to the less well-known Cavia rufescens Lund (Lund 1841, Waterhouse 1848, Thomas 1901). This cavy is considerably smaller than Cavia aperea or Cavia porcellus, both in total size and in the individual bone measurements. Thomas asserts that Cavia rufescens neverreaches the size of Cavia aperea. The color is agouti or "ticked," as in most wild rodents, but somewhat darker than the agouti of Cavia porcellus, because more black shows in the individual hairs and less yellow on their subapical bands. The belly varies from a light yellow to a slightly ticked condition. The systematists lay great stress on the formation of the last upper molar, in which a deep, narrow indentation on the outer surface almost separates the small third lobe from the body of the tooth. Lund describes his specimen from Minas Geraes, Brazil. In all essential points the wild animals in this experiment agree with the descriptions, plates, and general locality given by the above-mentioned authors.
A report of the experimental work does not necessitate an argument on the number of differential characters which would infallibly place two types in those more or less arbitrary categories—"species." It is sufficient for the purposes of this problem to find that the wild cavies used belong to a species more distantly related to the tame guinea-pig than are Cavia aperea or Cavia cutleri, according to the methods of most taxonomists. The taxonomists differ much among themselves. For instance, Waterhouse held that Cavia porcellus, Cavia aperea, and Cavia cutleri might all be placed in the same species. He found forms bridging typical differences. Darwin (1876) held that Cavia aperea was not the ancestor of the guinea-pig, basing his views on the fact that a distinct genus of lice infested each form. As far as his evidence goes, it might be considered decisive, for entomologists have reported that closely related mammals are infested by closely related lice (Osborn 1908). Giebel (1855) placed a number of cavy forms in the species aperea, and held that Cavia rufescens was only a variety of the larger Cavia aperea. Nehring (1889) considered Cavia cutleri to be the direct ancestor of our tame guinea-pig, being inclined to such a view on both historical and morphological grounds. He later showed (Nehring 1893, 1894) that Cavia aperea may be reciprocally crossed with the guineapig and give perfectly fertile offspring—fertile inter se or when mated back to either parent. Thomas (1901) is in doubt as to which of the two wild forms, Cavia aperea or Cavia rufescens, is the real ancestor of the guinea-pig. It would appear, from a comparison of Nehring's experiments and the experiments described in this paper, that Cavia aperea must be more nearly related to the guinea-pig than Cavia rufescens is, for the latter gives sterile male offspring in a cross with the tame guinea-pig, whereas Cavia aperea does not.