In men and other animals there is little red flesh covering for the brain, the spine and the great ganglia of the nerve machinery. So many animals like the lion and bison have manes as an extra shield for the nerve centres. {14} The human head and neck, for instance, grow hair, not to encourage barbers, but for the prevention of sunstroke, and this varies in colour with the degree of sunlight. So all natural breeds of horses have a dark forelock and mane with a streak of strong brown or black colour from the withers to the root of the tail, thus guarding the whole length of the spine. This armour and shield defines for us two primitive types: The Bay of the Desert produced in fierce light the year round. The Dun of the Steppes produced in fierce light limited to the summer. [Sidenote: Protective colour] And here the need of clear thought leads to a new definition of "protective" colour. The dun Siberian tiger, largest and fiercest of all cats, hunted the Dun pony of the Steppes. The dun lion of Africa hunted the Bay horse. Had both cats and both horses been painted sky blue, their relative chances in the chase would be exactly the same. They do not owe meat or safety from attack to their body colour. Both species would have perished under the actinic rays of sunlight but for their equal shield of non-actinic colour. The purpose of body colour is defence from actinic light. Only the markings are {15} protective as concealing animals from one another. So far I have not been able to find in books about horses these applications of facts accepted by men of science, which are of use to horsemen. In the light of such evidence the close hogging of horse's manes needs reconsidering. PART II. THE GREAT ICE AGE. [Sidenote: The great Ice Age] Unless a fellow can swim he has no business to go out of his depth; but if he minds his business, he loses all the fun. It is the application of these two principles which leads me to a problem in the history of the horse which nobody has solved. The species is native to the Americas, where it became extinct. One theory of this extinction imagines a germ, like that of horse-sickness, whose range covered all latitudes from tropic to sub-arctic. Such a hearty microbe as that would seem unusual. The other theory relates to a disagreeable change in the climate, which overwhelmed the drainage basin of the North Atlantic with a field of massive ice. That seems conclusive until one reflects that the Pacific slope of the United States and the continent of South America remained as warm as ever. The cold {16} of the Great Ice Age does not explain the wiping out even in North America of the camel, elephant, tapir and horse. [Sidenote: The Ice Age]