MAMMALS 75 



rapid, and takes place chiefly in July and August. When 

 the antlers have reached their full size the " velvet " is shed, 

 and this takes place between August 20 and September 

 15. The size of the antlers and the number of branches or 

 points increases every year until the vigour of the stag begins 

 to decline from old age. 



It is clear, therefore, that there is a periodical mechanical 

 irritation of the antlers corresponding to their periodical 

 development. They are strained and rubbed in the fights of 

 the stags, and these fights take place only at a certain period 

 of the year lasting not more than three months. We know 

 that irritation of a bone by blows will cause exostosis, and 

 we may therefore consider it probable that the growth of the 

 antlers was caused originally by the ancestral stags butting 

 their heads together, and so irritating the frontal bones. 

 When the fighting ceased the bony outgrowths so caused were 

 shed, and when the rutting season came on again, the fights 

 and irritation of the bones were repeated, and the exostosis 

 or outgrowth of bone was reproduced. 



This hypothesis harmonises well with the important fact 

 of the increased development of the antler in successive 

 years. For there is a tendency for any physiological modifi- 

 cation to become either periodical or chronic ; that is to say 

 a process once set up by a certain irritation is more easily 

 produced the second time than the first, and the more often it is 

 produced the more energetic its action, and the more exten- 

 sive its results. The hypothesis also harmonises with the 

 known connection between the development of the antlers 

 and the normal condition of the testes. If a stag is castrated 

 the antlers are not fully developed again. The growth of the 

 antlers is of course in existing stags an inherited process ; it 

 is independent of any mechanical stimulation in the in- 

 dividual, but is associated inseparably with the maturation 

 and periodical activity of the testes. 



