460 GENETICS IN RELATION TO AGRICULTURE 



are contained in definite arrangement in a fixed number of chromosomes. 

 So far as our knowledge goes the chromosome mechanism behaves 

 in a perfectly typical fashion. It is possible, therefore, to state with 

 some degree of confidence that the mechanical details of recombination 

 are the same in the higher animals as in the lower the conception is a 

 universal one. 



But it is perhaps necessary to enquire whence come the germinal 

 elements which are the basis of the great diversity of characters exhibited 

 by domestic animals. All breeds of the northern cattle are interf ertile ; 

 they appear to belong to a common, related group ; and not only that but 

 there are evidences of relationship of this great group with that other 

 great group of humped cattle of the Orient, the zebus. Diversity among 

 horses and sheep is not less striking than among cattle, and even in 

 swine it is very great. So far as present evidence indicates most of 

 this diversity is a consequence of polyphyletic origin; for from the be- 

 ginning of domestication, man has constantly taken his livestock with 

 him in his wanderings, and has allowed them to mix with whatever other 

 types of the same species they might come into contact. 



Although a unanimity of opinion by no means obtains as to the 

 path of descent of modern horses, the evidence of some kind of poly- 

 phyletic origin may be regarded as conclusive. Ewart, who has given 

 a good deal of attention to the problem of the origin of domesticated 

 animals, inclines to the belief that the modern horse has sprung from the 

 intermingling of several wild species which may have been connected 

 with the three-toed horse of the Miocene period by different lines of 

 descent. These wild species may be broadly characterized by their 

 adaptability to different habits of life as horses of the forest type, of 

 the plateau type, of the steppe type, and of the Siwalik type. Horses 

 of the forest type were for the most part small, probably of a fawn 

 color, and richly, although not conspicuously, striped. They were 

 adapted to life in the forests and had very definite characteristics which 

 are still seen in some of the modern breeds of horses. Fairly good repre- 

 sentatives of this type are met with among the ponies of Scotland and 

 Iceland, and other Eurasian regions. Evidences of forest ancestry are 

 indicated in some modern breeds such as some Arabians and many of 

 the modern breeds of draft horses such as the Suffolk, and in the 

 Shire, Clydesdale, and Percheron to a certain extent. The plateau 

 type of horse comprises many different races, but in general they 

 are all finely built, slender limbed, fleet ponies of which modern 

 representatives are the Celtic ponies of the British Isles and Mexican 

 ponies. Arabs and Barbs and through them the modern Thoroughbred 

 are largely of plateau ancestry; and there is considerable evidence of 

 the same blood in some Shetland ponies. The steppe type of horse, 



