82 CHILDHOOD OF ANIMALS 



comparison too closely to insist that the markings of modern 

 mammals were the actual relics of lost scales, but the presence of 

 scales and of spots and reticulations and stripes (the latter being 

 lengthened spots or rows of spots that have fused) maybe similar 

 expressions of the nature of the external covering of the body. The 

 skin is not a continuous sheet, uniformly stretched over the body, 

 but is a composite structure growing from many centres, supplied 

 by different nerves and blood-vessels, and may well reveal this 

 composite character by a tessellated appearance, retaining this 

 where it is useful, or harmless, as in the well-guarded young, or in 

 the least conspicuous parts of the body of adults, or have it obliterated 

 where it is harmful. 



The possibility of changing the pattern and colour of the exterior 

 of the body comes about in mammals and birds because these for 

 other reasons are able to moult. Animals have to contend not 

 only with the larger kinds of foes which are able to pounce on them, 

 kill them and devour them bodily, but with a multitude of minute 

 enemies which harbour on the outside of their bodies and injure their 

 health in many ways. The spores of bacteria and moulds are rained 

 on them from the dust of the air, are rubbed on them by contact, or 

 are floated on them in water. Fleas and bugs, lice and ticks, a 

 prolific swarm of hungry vermin, provided with biting and sucking 

 organs, grasping hooks, claws with adhesive pads, and devices 

 innumerable for maintaining their position if once they reach the 

 body, assail them. These parasites may do much or little direct 

 harm ; sometimes they feed only on the waste secretions of the body 

 and do little more than cause a tickling irritation ; sometimes they 

 burrow deeply, or scratch and gnaw until they produce serious 

 wounds, or weaken their host from direct loss of blood. Still 

 more often those that are blood-suckers do damage, not only directly, 

 but by introducing the seeds of diseases that may be fatal, 

 carrying them from infected to healthy animals. Many of the 

 soft-skinned lower animals, such as newts and frogs, and still more 

 worms and slugs, whose bodies would otherwise be a ready prey, are 

 protected from the attacks of external parasites by their power of 

 producing slimy secretions which float off the lintruders before these 

 have time to establish themselves. Other animals keep their skins 

 clean by a process acting like the scaling paints with which the sub- 

 merged parts of warships are covered, to prevent their bottomsrgetting 

 fouled with barnacles. The outer horny layer of the skin is constantly 

 shed off, carrying with it many of the parasites and leaving the 



