Respiration in Invertebrate Ajiimals. 405 



organs and the process of breathing in the Lamellibranchiate 

 classes have engaged the study of various able observers^ no 

 special attention has at any time been given to the minute and 

 ultimate anatomy of these parts in the higher Mollusca. General 

 views can only securely rest on correctly ascertained particulars. 

 The laws which govern the structure and functions ojp individual 

 organs of animals may be discovered with far gi'eater certainty 

 by tracing the advancing phases of their histological elements 

 throughout the zoological series, than by following the mutations 

 w^hich occur in the progress of their embryonic development in 

 one class. Comparative anatomy is still very deficient in such 

 information. The general anatomy of organs has been ably 

 written : their histological anatomy remains untold. The in- 

 formation to be drawn from the serial history of any given 

 structural element of any given complex organ is more calcu- 

 lated to illustrate the homology of that organ and the archi- 

 tectural design under which it has been built, than any know'- 

 ledge which can be attained by a descriptive account of its 

 mere general conformation. The inmost constituent of struc- 

 ture may display greater invariableness of character than external 

 outlines and the gross configuration. It is only by such a serial 

 history that that which is essential to an organ can be eliminated 

 fi"om those superadded and accessoiy parts which are separable 

 and non-essential. For instance, it is quite certain (see a memoir 

 on the Serial Histology of the Liver (under the title of the Phy- 

 siology of Cells) by the author, in Guy's Hospital Reports for 

 1848), that the same idea of the liver cannot be acquired by 

 tracing the stages through which the organ passes in the em- 

 bryo of the mammal, as that which is obtained by exhibiting 

 consecutive pictures of the form under which it occurs in the 

 successive links of the animal chain. There is much that is 

 deceptive and fallacious in general resemblances and leading 

 analogies. The ultimate and the particular must be seized before 

 the comparison of general conditions can become correct and 

 complete. How far-sought and really worthless it were to assert, 

 that a single cell of the mammalian lung finds its counter- 

 part, its prototype, in the membranous vesicle on the dermal 

 surface of an Asterias, in which the process of respiration is 

 carried on ! In such an alleged analogy how many real deep 

 differences are ignored ! What a wide space is overleaped by 

 simplicity and ignorance ! How- unsafe are generalities in 

 science when unsupported by ultimate knowledge, by facts and 

 details ! On the other hand, how utterly valueless are figures, 

 facts, and particulars, unless they form the substratum of some 

 generality ! To ap])ly them to the maintenance of any theory 

 invests them with life, renders them mutually coherent. Aban- 



