xiv EDITOR'S INTRODUCTION 



morphology makes contact with physiology, but lack of space 

 forbids a discussion of this point here. 



Finally, there is the bearing on morphology of functional 

 modification. Most biologists do not seem to realize 

 the extent to which functional modification occurs in the 

 normal vertebrate body. It appears to be true, not only that 

 the size of every muscle in the body depends upon function, 

 but the size, direction, and structure of every tendon and 

 bone ; the detailed conformation of the blood-system depends 

 largely, or perhaps wholly, on hydrodynamic considerations ; 

 the size of every gland is regulated by its function ; and 

 even the nervous system does not escape. It is only in 

 earliest development that structure precedes function : later, 

 structure is the resultant of function. 



The recognition of these facts demands a new attitude 

 towards the genetic and evolutionary bases of structural change. 

 To take but one example, it is disturbing but true to find 

 that the differences in form and minute architecture of the 

 human heel-bone which distinguish it from that of apes are 

 due to functional modification in each generation — to the fact 

 that we put our weight on it in a different way owing to our 

 walking upright. It is also disturbing to realize that in other 

 groups, function does not play this important role in mould- 

 ing structure. In all holometabolous insects, the size and 

 form of all hard parts come into being once and for all, 

 without previous function, since they have not existed in the 

 larva, or been used in the pupa, and without the chance 

 of being later modified by function, since there is no further 

 moult. Thus definitive form — the morphologist's raw 

 material — is arrived at by quite a different method in the 

 two highest groups of animals. 



I have, I hope, said enough to show that certain aspects of 

 vertebrate morphology will bear restating ; and Mr. de Beer's 

 pages are themselves the best evidence of his success in 

 achieving that restatement without abandoning any of the 

 essentials which give morphology such value as a discipline 

 in its own right. 



JULIAN S. HUXLEY. 



