104 Mr. J. D. Dana on Zoophytes. 



poses take place by variations in corresponding sets of organs 

 or parts of organs, the relations produced are termed homological ; 

 and the relations are analogical when they depend on a simi- 

 larity of function, however produced*. 



As the several families or classes of animals are exposed, in 

 some respects more or less general, to the same circumstances, 

 they would naturally undergo, in many instances, either homo- 

 logical or analogical modifications, occasioning that serial paral- 

 lelism alluded to on a preceding page. And again, as the animals 

 of the same class may be fitted to many different circumstances 

 in nature, other parallelisms should exist, of a wider character. 



The order in which the above sources of distinctions in the 

 animal kingdom are mentioned, may be in the main nearly that 

 of their relative importance. Yet it is well known that a set of 

 characters valuable in one group is worth nothing in another : and 

 this holds true in some cases even with those characteristics that 

 are in general fundamental. It seems at first a violation of all 

 propriety, to arrange together animals having gills, and those that 

 have none ; those that have a heart, and those that are destitute of 

 even a vestige of one beyond a distant valve or two in the circu- 

 lating system ; those that have distinct arteries, and those whose 

 arteries are only the lacunal passages among the muscles and 

 other organs. Still this may be in accordance with a philo- 

 sophical classification. The class Crustacea actually illustrates 

 each of the three anomalies just stated. If the singular Am- 

 phioxus is truly a fish, as many ichthyologists affirm, we may 

 have a vertebrate animal without a brain, and without a sense to 



* Prof. R. Owen, the eminent comparative anatomist of England, mentions 

 three kinds of homology, viz. " general," " serial," and "special." " General 

 homology is the relation in which a part or series of parts stands to the ideal 

 or fundamental type ; and thus, when the basilar process of the occipital 

 bone in Anthropotomy is said to be the • centrum ' or ' body of the last cra- 

 nial vertebra/ its yeneral homology is enunciated. When it is said to 

 repeat, in its vertebra or natural segment of the skeleton, the body of the 

 sphenoid bone, the body of the atlas, and the succeeding vertebral bodies or 

 centrums, its serial homology is indicated. When the essential correspond- 

 ence of the basilar process of the occipital bone in Man with the distinct 

 bone called ' basi-occipital' in a crocodile or a fish is shown, its special ho- 

 mology is determined." — Phil. Mag. xxviii. 3rd ser. 526, June Supp. 184(5. 



We refer the reader also to a very excellent paper " on the Structural 

 Relations of Organized Beings," by H. E. Strickland, F.G.S., Phil. Mag. 

 xxviii. 3rd ser. 354, in which the subject of affinities in organic beings is 

 presented in a clear and philosophical light. In addition to the terms 

 homology and analogy, Mr. Strickland proposes the word iconism, to ex- 

 press resemblance of form without a similarity of structure or function ; for 

 example, the resemblance of the flower of the pea to a butterfly, or the shell 

 Haliotis to an ear ; and it includes also resemblances between species arising 

 from accidental coincidence of colour ; while analogy includes such resem- 

 blances as depend upon a similarity of function. 



