CONCLUSIONS 483 



the antlers of the little Muntjack are relatively smaller than 

 those of the larger Red deer. There are of course exceptions 

 and complications, but from the present point of view, the main 

 thing to notice is that for an organ which shows heterogonic 

 growth to appear at all, the animal's body must have reached 

 a certain absolute size. Now as the different races of Titano- 

 theres evolved, their size increased, in common with nearly 

 all the groups of mammals. Independently, each of these races 

 of Titanotheres developed bony knobs on the skull, and as the 

 size-increase of the animals continued, the bony knobs became 

 relatively larger still. The bony knobs are heterogonic organs, 

 and their independent appearance in different races is not due 

 to any directive force, but automatically to the increase in size 

 of the body of the animal. This increase of body-size was 

 probably due to random variation selected by natural selection 

 in the direction of greater size because it is (up to a point) 

 advantageous, and has survival value. From the common 

 ancestor of the different races it is only necessary to assume 

 that the capacity was inherited to produce bony knobs if and 

 when a certain body-size is reached. On this view, therefore, 

 " Orthogenesis " does not mean directed evolution, but merely 

 directional. It also enables an explanation to be given for the 

 cases of extinction of animals in which the size of the hetero- 

 gonic organ (consequent on the large size of the body) had 

 become so great as to reduce the animal's chances of survival. 

 This applies to the Irish elk, which was a very large deer with 

 relatively immense antlers. 



Attention must now be paid to the terms " primitive " 

 and " specialised," which were defined early in this book, and 

 which have been consistently used throughout. In the first 

 place, it is necessary to notice that their meaning is relative, so 

 that it is possible to find an animal which is primitive when 

 compared with one and specialised when compared with another 

 animal. A specialised animal is one which is committed to a 

 particular line and so has a restricted potency of evolution. As 

 a rule, specialised animals are adapted to a particular mode of 

 life, and this adaptation has entailed either the development or 

 loss of certain structures which render the animals unfit to live 



