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 interfertile; 
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 1s 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, 
Digitized by Microsoft® 
