100 INITIATIVE IN EVOLUTION 



or projecting slightly to one or other side, as we saw in the case of 

 the cow. The fore legs are obviously in no way affected as to the 

 direction of the hair in the sitting posture, and the hind legs, being 

 doubled up and subject to the direct downward weight of the body, 

 are also free from the sliding pressure, which we shall see affects 

 the fore limb when the dog lies prone. Thus of the three supports, 

 fore legs, hind legs and tuberosities of the ischium, two are necessarily 

 unaffected in their patterns of hair. The anatomical conditions 

 of his tuberosities are very different in this respect. They are 

 covered with a large slippery bursa just beneath the thick skin, 

 and the slightest movement of this alert and restless animal, even 

 of his head, conveys to this region a small change of position. He is 

 virtually like a sick person on a water or air cushion, and we all 

 know how very small movements of the body are felt in a slight 

 stirring of the supported parts by these. The effect of this is that 

 the hair over these bursae is seldom at rest from external or extra- 

 neous forces, to say nothing of its own imperious constant growth 

 of one incn in two months. In Fig. 38 one sees the hair-stream 

 curving round the buttocks towards the region of these bursae, 

 and trying to reach the middle line. It meets with so much 

 opposition that the very conditions for producing a reversed area 

 are present and the result is just what one would expect to find. 

 The pattern is formed exactly over the bursae limited to this area, 

 and it does not expand anywhere because there is no need for it to 

 do so. So when one observes on the surface just below the tail a 

 pattern, often in a black-and-tan terrier marked by a tan patch of 

 hair, one reads the record of the long time spent by the dog in 

 sitting as he meditates on some fresh or past escapade of " A Dog's 

 Day." 



The statement just made that the hind leg does not share in 

 the effects of pressure is not strictly correct ; it applies to the leg 

 properly so called. But the upper part of the thigh exhibits a 

 very clear reversal of hair due to the weight of the body acting here 

 against the streams from the side of the thigh, which are seen 

 endeavouring to make their way to the inner side. They are 

 arrested by a long ridge of hair which marks the obstacle presented 

 by the weight of the body acting here. This completes the story 

 of the way in which sitting affects the hair of the dog, and is 

 shown in Fig. 38. 



Lying Attitude. 



There are four attitudes adopted by the dog in lying. In 

 the first, when he sleeps he lies stretched out on his side on 



