1034 THE THEORY OF TRANSFORMATIONS [ch. 



widely current and more complicated hypotheses of biological 

 causation. For it is a maxim in physics that an effect ought not 

 to be ascribed to the joint operation of many causes if few are 

 adequate to the production of it : Frustra fit per plura, quod fieri 

 potest per pauciora. 



We might suppose that by the combined action of appropriate 

 forces any material form could 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 transformation of 

 the simple germ into the complex organism. But we need not 

 let these 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, 

 coordinates shall constitute an harm<5ni6us and more or less sym- 

 metrical system. We should fall into deserved 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, both by our method 

 , and 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. For it is a grave sophism, in natural history as in 

 logic, to make a transition into another kind*. 



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

 himself laid down when, in defining a "genus," he shewed 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 mag- 

 nitude, 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 saiiie 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 



* The saying heterogenea comparari non possunt is discussed by Coleridge in his 

 Aids to Reflexion. 



