IN THE LIMBS OF MAMMALIA. 59 



of Nature, and which seem to indicate that here is one and the same thought of the Creator 

 expressed in terms geometrical, morphological, physiological, and zoological. 



From what has been said concerning the axes of the three higher types, it will appear 

 that they all exist in each of the types, just as they would in a geometrical sphere, variously 

 compressed ; but that this presence of all must be carefully distinguished from the prominence 

 of one axis in the type characterized thereby. Thus the laterality of Mollusks is something 

 more than the bilaterality or bilateral symmetry which exists also in the Articulates and 

 Vertebrates ; the cephality, or more properly the longitudinality of the latter type must be 

 distinguished from the antero-posterior symmetry so evident in certain Articulates ; and the 

 tergality, or more properly vertically of these, from any resemblance, if it should be observed, 

 between the dorsal and ventral regions of the Mollusca ; even the radiality of the lowest 

 type must not be confounded with the appearance of it in certain Mollusks above mentioned. 



But, in fact, mere resemblance between any two regions of the body is not what we desire 

 to express ; for, although it may exist, there is quite as often a striking contrast, as in the 

 Gasteropoda among Mollusks, and in Man with most Mammalia among Vertebrates. Our 

 term should refer rather to the direction of the axis which is specially prominent in any 

 one type, at the poles of which are the two regions which are the external evidence of this 

 prominence, and which morphologically repeat each other, but as has been instanced, may 

 be teleologically most diverse ; and therefore if radiality and laterality are accepted for the 

 two lower types, (for despite our morphological equality of Mollusks and Articulates, the 

 latter are physiologically the higher of the two,) the corresponding terms for the other types 

 are verticality and longitudinality : for tergality and cephality only express the prominence of 

 the region at one end of the axis over that at the other, a prominence which is teleological, 

 while our idea is strictly morphological. Radiality, laterality, verticality, and longitudinality 

 are, morphologically, peculiar to and characteristic of Radiates, Mollusks, Articulates, and 

 Vertebrates respectively, but, teleologically, they may be as it were ingrafted upon the 

 others in the shape of radiation, bilaterality, tergality, and antero-posterior symmetry. 



In accordance with the law of polarity, we shall expect to find the organs at the two 

 poles of a prominent axis morphologically similar, and thus to observe relations of polar 

 homology between them, — of lateral homology in the Mollusks, of vertical homology in the 

 Articulates, and of longitudinal homology in the Vertebrates ; all existing more or less in 

 each of these three, but specially prominent one in each ; while in the Radiates there is the 

 radial homology so early recognized between the four or five diverging segments. These 

 four kinds of polar homology are the most general. 



We come now to consider more particularly that relation of homology which is charac- 

 teristic of the Vertebrates, and included in the idea of longitudinality, otherwise expressed 

 by the compound term antero-posterior symmetry ; both these terms are objectionable 

 on the score of length, but the former is really significative of the idea we mean to convey. 



It may be well at this point to find how fully our deductions are borne out by the facts. 



It is fortunate indeed when our views a priori are confirmed by our investigations a 

 posteriori, so that we safely ascend and descend the hill of science, without being dazzled 

 and led astray by glittering and apparently correct generalizations, or losing our way 

 amidst the labyrinth of facts, and, in our eagerness to advance, in place of steadily looking 

 for the landmarks Nature has provided, becoming impatient and exhausting our strength 

 in cutting down or rooting up, if possible to destroy, whatever stands in our way, or perhaps 

 using it to bridge over gaps in our self-made path which never would have existed, if, at 

 the outset, our aim had been to discover, not to create, to learn from God, not to be teach- 

 ers ourselves. 



