INTERNAL CAUSES OF VARIATION 157 



not regenerate, and in their place there is only a "cartilaginous 

 tube attached to the broken vertebra." 1 



In the first case (that of the salamander) cell division, which 

 would normally remain suspended through life, was able upon 

 occasion not only to resume activity but also to begin back at the 

 proper point in ontogeny 2 and repeat its normal processes from 

 that point onward. Moreover, in this particular instance it can 

 do this not once but many times. 3 In the lizard, on the other 

 hand, regeneration is not complete, as no true vertebrae are 

 formed. Higher animals generally have but slight powers of 

 regeneration, but all have enough to repair ordinary injuries to 

 the skin, bone, nerves, etc., showing that the power of cell 

 division is not entirely lost at maturity; in other words, that 

 cessation of growth when the normal size is reached is due to 

 some cause other than the failure of the power of cell division. 

 There are many cases of abnormal size of certain parts due to a 

 failure of this process to arrest itself at or near the proper 

 point. "Big heads," "giant kidneys," and similar pathological 

 cases are instances in point, but whether the division is mitotic 

 or amitotic has not, so far as the writer is informed, been 

 determined. 



While the limitation of cell division can certainly be influenced, 

 especially by the food supply and by exercise, it is manifest that 

 its absolute control is, and doubtless always will be, largely beyond 

 our power. All animals get feed enough to more than build their 

 bodies, and the point at which growth ceases seems to be mainly 

 constitutional. If we could regulate size directly, it would vastly 

 simplify the process of breeding, but as it is now, we are obliged 

 to " breed for size " and feed accordingly. 



1 Morgan, Regeneration, p. 198. 



2 Ontogeny refers to the development of the individual, as phylogeny refers to 

 that of the race. 



3 The absolute limits of regeneration are not known. Speaking generally, they 

 are high in plants and low in animals. The salamander has been known, however, 

 to restore tail and all four legs six successive times (Morgan, Regeneration, p. 5). 

 The deer grows a new set of antlers every year. This is hardly a case of regener- 

 ation, however, because successive growths are each more complicated than the 

 former, each adding its characteristic prong; but it is a good instance to show the 

 persistence of the power of cell division. 



