EXCESSIVE GROWTH 409 



the size of the organ gets too great to be any longer compatible 

 with the well-being of the race, natural selection again steps in 

 and eliminates the race. The same argument of course applies 

 to the size of the body as a whole as well as to that of its con- 

 stituent parts. 



It may be thought that many of the bizarre and almost 

 monstrous characters under discussion, such, for example, as 

 some of the excrescences of the dermal armature in extinct 

 reptiles (Fig. 145), can never have had any value as adaptations, 

 and that therefore natural selection could never have encouraged 

 them to increase so much in size as to get beyond her control. 

 Here, however, the principle of correlation comes in. Just as 

 many totally different organs are affected by disease of the 

 pituitary body, so the removal of the gland which controlled the 

 development of some undoubtedly useful organ, such as a frontal 

 horn, might at the same time permit the growth of all sorts of 

 excrescences which have no adaptive significance. 



Thus it appears not impossible that, the normal checks to 

 growth being removed along certain lines by the action of natural 

 selection, a definite direction might be given to the course of 

 evolution, which the organism would continue to follow irrespec- 

 tive both of natural selection and of the principle of use and 

 disuse. 1 



In the present state of our knowledge, however, the above 

 suggestions can only be regarded as tentative. They are doubtless 

 open to much criticism, and it is unfortunately impossible to 

 subject them to the crucial test of experiment. 



1 I have discussed this question at somewhat greater length in a paper read 

 before the British Association for the Advancement of Science (vide Report of the 

 Portsmouth Meeting, 1911). 



