482 CONCLUSIONS 



Stegocephalians, which were not on the line of descent of the 

 reptiles, nevertheless show a number of changes in evolution 

 which took place parallel to those which were going on in their 

 " cousins " the reptiles. 



The answer must be that the " fathers " and the " uncles " 

 inherited something from the " grandfather " which deter- 

 mines the course of their evolution. This something need 

 not, however, have been visible in the " grandfather," so that 

 the " fathers " and the " uncles " in which the something does 

 become visible appear to have evolved it independently. In 

 these cases there appears what may be called a latent homology 

 between the structures in question, and which accounts for the 

 so-called " Orthogenesis." In any case, it is most important 

 to avoid the impression that " Orthogenesis " implies a purpose- 

 ful or directive force, or that evolution takes place in straight 

 lines. Such impression is quickly dispelled by a consideration 

 of the record of success and failure of the different groups of 

 animals during evolution. If a directive force were responsible 

 for evolution, it would seem to be peculiarly malicious, for 

 most groups of animals have been " directed " to their doom 

 by extinction. 



An insight into what " Orthogenesis " really means is 

 given by a study of the relative sizes of parts of animals to the 

 whole animals, at different absolute sizes. It is found, for 

 example, that the size of the antlers in Red deer is relatively 

 larger in large animals than it is in small ones. That is to say, 

 that the larger a Red deer grows, the relatively larger do its 

 antlers become, on the average. These cases are susceptible 

 of mathematical treatment, and it is found that the antlers not 

 only grow faster than the body, but they grow faster at a 

 constant rate, for the ratio of the growth-rates of antlers and 

 body remains constant. Organs to which this principle applies 

 are called heterogonic, and Heterogony is of wide occurrence 

 in the horns and bony nobs of various groups of Ungulate 

 mammals. Now just as the heterogonic organ is relatively 

 smaller in small animals, it is found as a rule that in two 

 species of one genus both of which possess this organ, the larger 

 species will have the relatively larger heterogonic organ. So 



