366 PRINCIPLES OF ANIMAL BIOLOGY 



genus, cannot be crossed; that is, they cannot or do not produce hybrid 

 offspring. Other species may be crossed, but the hybrid offspring are of 

 low fertihty or even completely sterile. Some species, if crossed, produce 

 offspring only of the female sex, and these, since they are not partheno- 

 genetic, cannot give rise to a new type. 



What causes this sterility between species or in their hybrids is only 

 partially known. Difference in numbers of chromosomes is one obvious 

 cause, since there can be no complete pairing and meiotic division (page 

 195) of the chromosomes in a hybrid unless the chromosomes match. 

 Rearrangement of the genes in the chromosomes, such as turning one 

 segment of a chromosome end about, has a similar effect ; for in a hybrid 

 having two chromosomes alike in genes but differing in their arrangement 

 the pairing of the chromosomes is not normal. Many other chromosome 

 changes may occur. When an individual has two chromosomes of the 

 same sort in each pair, even if both are aberrant, it may behave normally; 

 and a group of such individuals may constitute a species. But when 

 they attempt to cross with individuals having chromosomes differently 

 constituted, abnormalities arise. Species are just as effectively isolated 

 by such chromosome changes as they would be if separated by a thousand 

 miles of ocean. Indeed, it is probable that separation of species from 

 one another is often rendered complete by such chromosome aberrations. 



Evolution of Domesticated Races. — One of the arguments used by 

 Darwin in favor of natural selection is the fact that animals and plants 

 under control by man have experienced enormous modifications. A very 

 few years of selection by man have produced observable changes in 

 cultivated plants, and herd records show similar though less striking 

 changes in domesticated animals. The method is selection. The l:)reeder 

 preserves individuals most nearly approaching his ideals in the belief 

 that they will transmit the desirable qualities, and sometimes they do. 

 Darwin concluded that all that was necessary to accomplish a similar 

 result in wild species would be some selecting agency to replace the 

 breeder. That selecting agency could not be endowed with reason or 

 foresight, but highly adaptive modifications could, he believed, be pro- 

 duced by selective action of the environment itself. The method, as 

 conceived now, has already been outlined. 



The written histories of domestic breeds do not go back far enough 

 to show the source from which any of our principal types of animals 

 came. Very early records show animals already in man's control, but 

 not much information about them is given. The sources of the various 

 animals have been conjectured from the qualities of breeds today and 

 the characteristics of wild species, but nothing is certain. The breeds of 

 poultry are believed to be descended from two mid sources, the jungle 

 fowl and the Malay fowl, both of the Orient. Egg-laying (juahties 



