326 CAUSES OF VARIATION 



parts than do those which are more highly differentiated. Still 

 the latter are not destitute of this faculty of replacement. The 

 teeth of many vertebrates are shed once and replaced ; rarely a 

 second replacement occurs. If the ox loses the horn, the loss is 

 permanent ; but the stag sheds his annually, each successive pair 

 arising from the same scar or bud, but each provided with an 

 additional prong. By what inherent quality of this particular 

 spot are we to explain this annual change in the character of 

 the part restored ? 



Birds shed their plumage, and many animals their hair, annu- 

 ally, as trees shed their leaves, and often the new growth dif- 

 fers materially from the old. The " milk teeth " are simpler than 

 the permanent set ; the color of the foal and the fawn changes 

 with maturity ; and the shape of the cotyledon gives little indi- 

 cation of what the real leaf will be. Nobody seeing the umbrella- 

 like first leaf of the basswood would suspect what the later 

 leaves will be, nor would one suspect the clover until the " third 

 leaf " appears. 



Repair of injury among higher animals seems to be exceed- 

 ingly limited. However, wounded muscles can " fill up " to some 

 extent ; cut nerves mend slowly ; severed blood vessels repair 

 themselves and restore circulation to the part ; liver, kidney, 

 glands, and tissues generally have sufficient power of regenera- 

 tion to close wounds and to replace lost portions more or less 

 perfectly, but nearly always with a scar ; broken bones will knit 

 or a small piece removed will be restored, but an entire bone cut 

 off will not be replaced. Of all parts the skin possesses the highest 

 power of restoration, probably because it is normally in active 

 growth from beneath to replace the parts worn off from above. 



The character of the regenerated part. The regenerated part 

 may compare with the original in any one of four different ways : 



1. It may be exactly like the original, as in the leg of a sala- 

 mander (holomorphosis). 



2. It may be like the original except smaller in size (mero- 

 morphosis). 



3. It may be different from the original but like some other 

 part of the body, as when an antenna replaces an eye (hetero- 

 morphosis). 



