6 On the present state of Zoology. 



easily accessible, and whose habits and economy are in consequence 

 almost entirely concealed from view. We find others possessed of 

 such a delicate and fragile structure, that all attempt to convey them 

 from their native spots, or to preserve them for future examination 

 and comparison, is rendered hopeless. These remarks are especially 

 applicable to the marine Invertebrata, with which our acquaintance 

 is as yet comparatively slight. How many multitudes of species 

 must there be immersed in the profound recesses of a deep ocean, 

 where no net or line ever reached. How many more, which, 

 if known, have been seen only at a casual glance, or at one period 

 of their lives, leaving us in utter ignorance of the details of their 

 history, and of the changes through which they pass in their advance- 

 ment to maturity. How many, like the Acalepha, can hardly be re- 

 moved from their native element, without entailing the entire disor- 

 ganization of their substance ; and if there be others which we suc- 

 ceed in preserving in our cabinets, how impossible it is, in most in- 

 stances, to effect the preservation of those parts, on which depend 

 perhaps their most important characters, and in ignorance of which 

 we may adopt the most erroneous views respecting their structure 

 and affinities. 



We shall be able to appreciate more fully the impediments to the 

 progress of Zoology arising from the sources last mentioned, if we 

 bear in mind two points of great consequence to be remembered. 

 One is the necessity of studying animals throughout their whole ex- 

 istence in order to become possessed of their true history. Not only 

 do all animals differ more or less in their young and their adult states, 

 but many, probably more than we are at present aware of, undergo 

 such great changes of structure, that, unless viewed in their transi- 

 tion forms, it were almost impossible to identify them as the same 

 species. There is no occasion to allude to the Batrachian Reptiles 

 or to the true Insects, in which these changes have been noticed from 

 the earliest times. But we may draw attention to the circumstance 

 that metamorphosis, to a greater or less extent, has been recently de- 

 tected in several groups in which it was not supposed formerly to 

 exist, and that therefore we stand in need of further observation to 

 say in what others it may not also occur. Without insisting on the 

 universality of this phenomenon in the class Crustacea, which can- 

 not be considered as established, and of which we shall have to speak 

 further presently, we may allude to those marked changes of form 

 which have been noticed in certain groups of that class by M. Milne-Ed- 

 wards,* and which, if not amounting to actual metamorphosis proper- 



* Ann. des Sci. Nat. 1835, torn. iii. p. 321. 



