EEPARATIVE POWERS OF HIGHER ANIMALS. 325 



requiring a higher degree of the stimulus of Heat, than does 

 their ordinary nutrition. In Lizards, an imperfect reproduc- 

 tion of the tail takes place when part of it has been broken 

 off; but the newly-developed portion contains no perfect ver- 

 tebrae, its centre being occupied by a cartilaginous column 

 like that of the lowest fishes. In the warm-blooded Verte- 

 brata generally, the power of reproduction after loss or in- 

 jury seems much more limited. We do not find that entire 

 parts or members once destroyed, are completely renewed; 

 though very extensive breaches of substance are often filled 

 up. The tissues most readily reproduced are Bone, the Simple 

 Fibres ( 22), and the Membranes (such as the Skin, the 

 Mucous and Serous membranes), of which these tissues form 

 the basis. As a general rule, losses of substance in Glandular 

 tissue, Muscle, and other parts of comparatively high organi- 

 zation, do not seem to be reproduced ; but there is a curious 

 exception to this in the case of Nervous tissue, which, with 

 Blood-vessels, is very readily re-formed in the new growths by 

 which losses of substance are repaired, as we often see in tha 

 rapid skinning-over of a large superficial wound. One of the 

 most remarkable manifestations of reparative power in the 

 Human body, is the re-formation of an entire bone, when the 

 original one has been destroyed by disease. The new bony 

 matter is thrown-out, sometimes within and sometimes around 

 the dead shaft ; and when the latter has been removed, the 

 new structure gradually assumes the regular form, and all the 

 attachments of muscles, ligaments, &c., become as complete 

 as before. A much greater variety and complexity of actions- 

 are involved in this process, than in the reproduction of whole 

 parts in the simpler animals ; though its effects do not appear 

 so striking. It appears that, in some individuals, this regene- 

 rating power is retained to a much greater degree than it is by 

 the species at large ; thus, there is a well-authenticated instance, 

 in which a supernumerary thumb on a boy's hand was twice re- 

 produced, after having been removed from the joint. In many 

 cases in which the crystalline lens of the eye has been re- 

 moved, in the operation for cataract, it has been afterwards 

 regenerated ; and there is evidence that, during embryonic 

 life, the regeneration of lost parts may take place in a degree 

 to which we have scarcely any parallel after birth ; attempts 

 being sometimes made at the re-formation of entire limbs, in 



