RECAPITULATION 223 



observed, but there is little reason to doubt that 

 these differences in scales and teeth correspond with 

 differences of mechanical stimulation. This does not 

 at all imply that the scales and teeth themselves 

 have been produced by mechanical stimulation, or 

 that the difference between the dermal denticles of 

 Elasmobranchs and the scales of Teleosteans corre- 

 spond to differences of stimulation. But the degree 

 of development of a structure whose presence is due 

 to gametic factors may very probably be modified 

 by external stimulation, and the modification may 

 become hereditary. If the views here advocated are 

 true, the two processes mutation and modification 

 must be always acting together and affecting the 

 development not only of the individual but of any 

 organ or structure. Thus the peculiarities of antlers 

 in stags, it seems to me, prove that the mechanical 

 stimulation due to fighting was the cause of the 

 evolution of antlers, that without the habit of fighting 

 in the males antlers would not exist. At the same 

 time each species of the Cervidae has its special char- 

 acters in the antlers, in shape and branching, and it 

 would be impossible to attribute these to differences 

 in mode of fighting : they are due to mutation. 



In connexion with the metamorphosis of Am- 

 phibia the case of the Axolotl has always been of 

 very great interest. In the few small lakes near 

 the city of Mexico where it occurs it has never 

 been known to undergo metamorphosis but is 

 aquatic throughout its life and breeds in that con- 

 dition. Yet in captivity by reducing tlie quantity 

 of water in which it is placed the young Axolotl can 

 be forced to breathe air, and then it undergoes 

 complete metamorphosis to the abranchiate con- 



