XVII] Tlffi COMPARISON OF RELATED FORMS 725 



the joint operation of many causes if few are adequate to the 

 production of it: Frustra fit per jplura, quod fieri potest per 

 pauciora. 



It is evident that by the combined action of appropriate 

 forces any material form can be transformed into any other: 

 just as out of a "shapeless" mass of clay the potter or the sculptor 

 models his artistic product; or just as we attribute to Nature 

 herself the power to effect the gradual and successive trans- 

 formation of the simplest into the most complex organism. In 

 like manner it is possible, at least theoretically, to cause the outline 

 of any closed curve to appear as a projection of any other what- 

 soever. But we need not let these theoretical considerations 

 deter us from our method of comparison of related forms. We 

 shall strictly limit ourselves to cases where the transformation 

 necessary to effect a comparison shall be of a simple kind, and 

 where the transformed, as well as the original, co-ordinates shall 

 constitute an harmonious and more or less symmetrical system. 

 We should fall into desej-ved and inevitable confusion if, whether 

 by the mathematical or any other method, we attempted to 

 compare organisms separated far apart in Nature and in zoological 

 classification. We are limited, not by the nature of our method, 

 but by the whole nature of the case, to the comparison of 

 organisms such as are manifestly related to one another and belong 

 to the same zoological class. 



Our inquiry lies, in short, just within the limits which Aristotle 

 himself laid down when, in defining a "genus," he showed that 

 (apart from those superficial characters, such as colour, which he 

 called "accidents") the essential differences between one "species" 

 and another are merely differences of proportion, of relative 

 magnitude, or (as he phrased it) of "excess and defect." "Save 

 only for a difference in the way of excess or defect, the parts are 

 identical in the case of such animals as are of one and the same 

 genus; and by 'genus' I mean, for instance. Bird or Fish." 

 And again : " Within the limits of the same genus, as a general 

 rule, most of the parts exhibit differences... in the way of multitude 

 or fewness, magnitude or parvitude, in short, in the way of excess 

 or defect. For 'the more' and 'the less' may be represented as 



