force capable of effecting the required transformation. Again, if such a simple alteration of the system of forces can be proved adequate to meet the case, we may find ourselves able to dispense with many 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 {725} the joint operation of many causes if few are adequate to the production of it: Frustra fit per plura, 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 ãshapelessã 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 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 whatsoever. But we need not let these theoretical conôÙsiôÙdeôÙraôÙtions 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 deserved and inevitable confusion if, whether by the mathôÙeôÙmatôÙiôÙcal or any other method, we attempted to compare organisms separated far apart in Nature and in zoological clasôÙsiôÙfiôÙcaôÙtion. 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 ãgenus,ã he showed that (apart from those superficial characters, such