EVOLUTION 299 



Wild species maintain species standards even in the face of 

 promiscuous cross-breeding. Breeds developed by artificial 

 selection can be maintained at a constant level only by breeding 

 from selected stock. Promiscuous or uncontrolled breeding 

 results in loss of breed characters and a tendency to revert to 

 the unimproved natural or wild type. Among pigeons an occa- 

 sional rock type will appear in an otherwise standard brood. 

 Such tendencies to revert to primitive types indicate that the 

 distinctive breed characters are not firmly fixed and that they 

 would not in all probability survive long in the wild state. 

 Indeed if domestic breeds are allowed to run wild they soon lose 

 their distinctive breed characters and tend to become similar to, 

 though not necessarily like, the original wild forms from which 

 they were derived. 



The results achieved by artificial selection in animal and plant 

 breeding show that the protoplasm of a species has a certain 

 amount of plasticity or capacity for modification, which can be 

 directed or controlled to a certain extent. Heredity is a con- 

 servative mechanism— it tends to keep species constant. Evolu- 

 tion is possible because the heredity mechanism permits a certain 

 amount of variability to appear in the offspring of similar parents. 

 Variability provides a background for evolutionary change. 

 Experience has shown that there are limits to the use to which 

 variations can be put in selective breeding, as will be pointed out 

 in later pages. 



Serum Tests. — The accuracy of the evolutionary relationships 

 of the members of a phylum or of any of its subdivisions, as 

 portrayed in the taxonomic system on the basis largely of mor- 

 phological data, may be put to a further test by means of certain 

 blood reactions. If human blood serum, i.e., blood minus cor- 

 puscles and fibrinogen, is injected in small quantities at repeated 

 intervals into the blood vessels of a rabbit, there are developed 

 after a while in the blood of the rabbit antibodies which, when 

 brought into contact with normal human blood serum, cause the 

 precipitation of the human blood proteins. Serum from the 

 blood of such an immunized rabbit may be called antihuman 

 serum. Blood sera of man, chimpanzee, gorilla, and monkeys, 

 at a certain dilution, all produce precipitates when mixed with 

 antihuman serum. At a higher dilution no precipitation is 

 obtained with monkey sera; and at still higher dilution only with 



