OTHER THEORIES OF EVOLUTION 335 



whether in fact the Machaerodonts did prey on the large 

 Pachyderms (there were plenty of smaller, more delicate 

 mammals to prey on), Matthew's theory does not account for 

 the fact that the series of their evolutionary history is pro- 

 gressive and that Smilodon, the Pleistocene representative, has 

 the largest and most ungainly canines. He may show that in 

 mid-Tertiary times there were plenty of Pachyderms of various 

 kinds for the Machaerodonts to prey on ; he docs not show that 

 in Pleistocene times the Pachyderms were of such a kind as to 

 necessitate the more exaggerated canines of Smilodon. 



Matthew (1910, p. 307) very rightly asks : ' How can a 

 race continue specialising in any particular direction beyond the 

 point when the specialisation is of use . . . the moment the 

 harmfulness of a character outbalanced its usefulness, a process 

 of elimination must act in weeding out the individuals in 

 which the character was most richly developed.' But it seems 

 to us that, even if the excessively enlarged canines may have 

 acted disadvantageously at the end of the series, Matthew 

 has not shown why they should have attained their excessive 

 size. We are quite ready to grant that, as soon as the canines 

 became inconvenient or definitely disadvantageous, the line of 

 the Machaerodonts might have been extinguished ; but we fail 

 to see why they should have been amplified and continued in 

 this stage in Pleistocene times, unless the Pachyderms also had 

 become more thick-skinned or more bulky, which is the very 

 thing Matthew fails to establish. 



(b) Both Loomis and Lang cite the remarkable growth of 

 the under- valve o£ Hippurites. This is a genus of Lamellibranch 

 molluscs which lived on coral reefs in the Cretaceous. It was 

 a sedentary form and its under-valve was, as usual, adherent to 

 the substratum. The valve was enormously thickened until it 

 formed a tubular structure sometimes afoot in length, the thin 

 upper valve lying on top like a lid. Lang, in discussing the 

 origin of this enlarged valve, has in mind only the protection 

 offered by the shell against the attacks of enemies. ' A shell 

 of half the thickness of a Hippurite shell is over-adequate 

 for protection.' But there is another possibility, and that 

 is that the tabular thickening of the lower valve is an 

 adaptive change, raising the mollusc above the encroaching 

 coral and reef-debris in the same way that many abyssal 

 animals and forms which live in silt are raised above it. 



