336 THE VARIATION OF ANIMALS IN NATURE 



However, it seems clear that this was not the true explanation, 

 for (i) Hippurites is never found with attached coral growth 

 on it and does not seem to have grown in such situations as 

 exposed it to this risk, and (ii) it seems sometimes to have been 

 orientated horizontally, so that in this position it was certainly 

 not growing upwards to escape the suggested danger. There is 

 a last possibility, suggested by the information given to us by 

 Mr. L. R. Cox : Hippurites is apparently found in clumps, like 

 a Vermetus or Rocellaria, and it is possible that the members 

 of such colonies grew to an excessive size to avoid overcrowding. 

 We certainly do not find such growth in recent colonial mol- 

 luscs, and the explanation just offered is not very plausible, as 

 the growth-habit is common to all the Hippurites. Some other 

 circumstance in the life of this mollusc may be ultimately dis- 

 covered which may suggest an adaptive explanation of the 

 growth of the under-valve ; but at present this seems unlikely, 

 and the suggestion that it is due to an uncontrolled production 

 of CaCO s is more plausible. 



(c) In the Babirusa the tusks grow first upwards, then back- 

 wards, and finally down towards the frontals, so that in some 

 individuals they pierce the face. That this is the effect of some 

 abnormal growth-process is suggested by the similar pheno- 

 menon in individual specimens of rodents. In the Common 

 Rabbit, e.g., the incisors are occasionally so excessively curved 

 that they turn over the maxilla and pierce it. Darwin (1901, 

 p. 792) points out a similar growth phenomenon in the old 

 males of the common Sus scrofa. He explained the abnormal 

 form of the upper canines of the Babirusa as fitted for defence. 

 6 Their convex surfaces if the head were held a little laterally 

 would serve as an excellent guard.' As Dendy {I.e. p. 1) says, 

 this is hardly a sufficient explanation of their enormous develop- 

 ment. Nor is it apparent why they should curve back to 

 guard the thick frontals. They certainly do not guard the 

 eyes. 



We are obviously dealing here with a series of facts concern- 

 ing which much that has been said in the chapter on Natural 

 Selection is applicable — viz. that the bionomic nexus involved 

 is unknown or incompletely known. We are dealing with 

 probabilities, and we have to weigh them in order to see which 

 are the more plausible. 



We agree that in the background of these phenomena there 



