XVII] THE COMPARISON OF RELATED FORMS 1035 



or parvitude, in short, in the way of excess or defect. For 'the 

 more' and 'the less' may be represented as 'excess' and * defect'*." 

 It is precisely this difference of relative magnitudes, this Aristotelian 

 "excess and defect" in the case of form, which our coordinate 

 method is especially adapted to analyse, and to reveal and demon- 

 strate as the main cause of what (again in the Aristotelian sense) 

 we term "specific" differences. 



The appUcability of our method to particular cases will depend 

 upon, or be further Hmited by, certain practical considerations or 

 qualifications. Of these the chief, and indeed the essential, con- 

 dition is, that the form of the entire structure under investigation 

 should be found to vary in a more or less uniform manner, after the 

 fashion of an approximately homogeneous and isotropic body. But 

 an imperfect isotropy, provided always that some "principle of 

 continuity" run through its variations, will not seriously interfere 

 with our method ; it will only cause our transformed coordinates 

 to be somewhat less regular and harmonious than are those, for 

 instance, by which the physicist depicts the motions of a perfect 

 fluid, or a theoretic field of force in a uniform medium. 



Again, it is essential that our structure vary in its entirety, or 

 at least that " independent variants " should be relatively few. That 

 independent variations occur, that localised centres of diminished 

 or exaggerated growth will now and then be found, is not only 

 probable but manifest; and they may even be so pronounced as to 

 appear to constitute new formations altogether. Such independent 

 variants as these Aristotle himself clearly recognised: "It happens 

 further that some have parts which others have not; for instance, 

 some [birds] have spurs and others not, some have crests, or combs, 

 and others not; but, as a general rule, most parts and those that 

 go to make up the bulk of the body are either identical with one 

 another, or differ from one another in the way of contrast and of 

 excess and defect. For ' the more ' and ' the less ' may be represented 

 as 'excess' or ' defect '|." 



If, in the evolution of a fish, for instance, it be the case that its 



* Historia Animalium i, 1. 



•j; Aristotle's argument is even more subtle and far-reaching; for the diflferences 

 of which he speaks are not merely those between one bird and another, but between 

 them all and the very type itself, or Platonic "idea" of a bird. 



