EFFECTS OF INBREEDING AND CROSSBREEDING. 3 



to reconcile with the results in the only other recent extensive exper- 

 iment on inbreeding of mammals, namely, those which Miss King 

 obtained with rats. It will be shown, however, that the two exper- 

 iments are complementary rather than contradictory. It may 

 be well to call attention to the excellent summaries of the present 

 state of knowledge on the subject to be found in Miss King's series 

 of papers and in " Inbreeding and Outbreeding" by East and Jones. 



PLAN OF THE EXPERIMENTS. 



An extensive investigation of the effects of inbreeding was planned 

 in 1906 by George M. Rommel, Chief of the Animal Husbandry 

 Division of the Bureau of Animal Industry. The work was com- 

 menced in that year at the Experiment Station of the bureau at Be- 

 thesda, Md., with guinea pigs as material. Since 1911 the experiments 

 have been carried on at the Experiment Farm of the Bureau of 

 Animal Industry at Beltsville, Md. Over 30,000 guinea pigs have 

 been recorded. The work has been conducted successively by Ralph 

 J. Carr, Dr. E. H. Riley, F. R. Marshall, and the writer. Essentially 

 the same system of records has been kept throughout. On taking 

 charge in September, 1915, the writer found the previous records 

 in excellent condition. In analyzing these and later data a great 

 amount of tabulation and calculation has been necessary. The 

 writer has been assisted successively by Walter J. Hall and Orson 

 N. Eaton, to whose painstaking care in this laborious work the carry- 

 ing through of the project is in a large measure due. All tabulations 

 and calculations have been carefully checked. 



HISTORY OF THE GUINEA-PIG STOCK. 



Dr. E. C. Schroeder, superintendent of the experiment station at 

 Bethesda, Md., has kindly furnished the following account of the 

 early history of the stock: 



The history of the station's stock of guinea pigs is as follows: When I took charge 

 of the experiment station of the bureau (at that time located at Benning Road and 

 Eighteenth Street NE., Washington, D. C), during the summer of 1894, I found on 

 hand about 250 to 300 guinea pigs, of all sizes and ages, about the history of which no 

 records were available. The general character of the animals indicated that some 

 attempts had been made to breed special varieties, such as curly haired guinea pigs, 

 white guinea pigs with black-smudged muzzles, long-haired guinea pigs, etc. 



As there was a superabundance of other work which urgently required attention 

 at the station, I at once abandoned all attempts to breed guinea pigs of special types 

 and kinds, and made of the breeding pens a strict business project, with no other 

 purpose in mind than the production of a sufficient number of satisfactory animals for 

 the technical work of the bureau. I used the stock on hand, eliminating the fancy 

 types as much as possible, because they are less satisfactory than the plain, vari- 

 colored, smooth-haired type for laboratory use. 



In the year 1895, as nearly as I can remember, I purchased a number of plain, 

 ordinary male guinea pigs, which, after a lengthy period of quarantine, were intro- 

 duced into the breeding pens. 



