SIRENIA. 197 



liim types or brandies ; the cnmJiiuafion of structure, as tlic test for defining' classes ; the complication 

 of structure, as evidenced in the dccjrce of orijnnimtion, as that for orders ; form as the character of 

 families ; ultimate structural peculiarities as those of genera ; relation of parts to each other and to 

 the workl around them, as those of species.* 



Although I quote this, I do not mean that I agree with all that Agassiz lays down on tlie 

 subject ; on the contrary, I dissent from the most of it. He believes in the existence of all these 

 subdi\nsions in nature. I do not. I see that organised beings exist in groups, but I see no two 

 groups bearing the same rank ; and no two equally well defined. It appears to me, therefore, that 

 the practice of natui'alists of which Agassiz complains, of using the terms class, order and 

 familj', loosely, and often interchangeably or indiscriminate^, is quite natural. How can one 

 use a term precisely for things which are never precise, but alwaj's irregular and uncertain ? 

 But at the same time, I think that most of his propositions, though not fomided on nature, will at 

 least prove useful for artificial arrangement. For instance, plan of structure is plainly at the 

 base of all arrangement, and the more nearlj' the plan on which different organic beings are 

 constructed corresponds, the nearer ought to be their place to each other ; such identity of plan 

 is the consequence of nearness of derivation from which all similarity of structiu'e proceeds ; and 

 altliough the difference between combination of structui'e and complication of structure, savours 

 more of refinement on words than of precise meaning, there can be little doubt that the general 

 principle involved in all his propositions is correct, viz., that before we come to employ the subor- 

 dinate objects or parts of structure, the more important modifications of its plan ought to be taken 

 into account. 



We should go against this principle if we placed the Sirenia in the same section as the Cetacea, 

 instead of ■vrith the Pachyderms, because we sliould then give weight to form in preference to plan of 

 structure. Because it is a swimming fish-shaped mammal, with the anterior limbs turned into fins, 

 and the posterior limbs absent, we should place it beside another swimming fish-shaped mammal, 

 similarly situated as regards limbs. It is the same mistake that used to be made long ago, when 

 the Seals, for the same reason, were jDlaced in the same group ; and the parallel to that ornithological 

 arrangement, objected to bj' many ornithologists, by which the water birds are placed all by them- 

 selves instead of being distributed among their congeners, according to their plan of structiu'c — the 

 Gidls beside the Raptores, and so on. 



There are, no doubt, one or two other indications of affinity wliich maj^ have weighed witli 

 Cuvier and Owen in inducing them to give the preference to the Cetaceous group as that to which 

 tlie Sirenia should be referred, such as, that the Rhvtina, one of the Sirexia, instead of liaving teeth 

 has horn-)' plates on the palate, suggestive of the whalebone of the whale ; that the cervical vertebra) 

 are only six in the Manatee, so far supplj'ing an ajiparcnt coincidence with the Wluiles, ^yhich, 

 sometimes from tlie effects of anchylosis, usually appear to have fewer cervical vertebras than seven ; 

 and that all the Sikenia have a broad transverse tail-fin like that of the Whales. 



On the other hand, the nostrils are not placed as in the Whales, nor do they serve as blow- 

 holes as in them, but they occupy the usual position in front of the muzzle. Tlie hirynx is that 

 of the Elephant, not that of the Whale. With the exception of tlie TUiytina, the form, structure, 

 and number of the teeth, are as in terrestrial pachyderms, and not as in the Whales, which have them 

 conical, numerous, and milike those of any other mammals ; further, in the Sirenia, the molar teeth 



* AoASSK, L., "Contributions to tlie Natural Ilistoi-v of the United States," vol. i. p. 137, et scq. Boston, 1857. 



