98 



REPORT 1873. 



TVERTEBnATA. J 



Amphioxys. 



rMOLI.USCA.") 



Conchifera. 



Tunicata. 

 QEchinodermata.J) (_^Arthropoda. J fMoLLirSCOIDA.") 

 Asteridffi. Crustacea. Pol)'zoa. 



Annelida. 



(^Vermes.j 



Platnidea. 



CP 



celenterat 



D 



Hydroida. 



Infusoria. Rhizopoda. 

 ('Protozoa. J 



Homology. 



In the comparison of organized beings with one another, certain relations of 

 great interest and significance become apparent between various organs. These 

 are known by the name of Homologies ; and organs are said to be homologous 

 with one another when they can be proved to be constructed on the same funda- 

 mental plan, no matter how different they may be in form and in the functions 

 which they may bo destined to execute. Organs not constructed on the same fun- 

 damental plan may yet execute similar functions ; and then, whether they do or 

 do not resemble one another in form, they are said to be merely analogous ; and 

 some of the most important steps in modern Biology have resulted from attention 

 to the distinction between Homology and Analogy, a distinction which was entirely 

 disregarded by the earlier schools. 



The nature of Homology and its distinction from Analogy will be beat under- 

 stood by a few examples. 



Compare the wing of a bird with that of an insect ; there is a resemblance be- 

 tween them in external form ; there is also an identity of function, both organs 

 being constructed for the purposes of flight : and j-et they are in no respect homo- 

 logous ; for they are formed on two distinct plans, which have nothing whatever in 

 common. The relation between them is simply that of analogy. 



On the other hand, no finer illustrations of Homology can be adduced than 

 those which are afforded by a comparison with one another of the anterior limbs of 

 the various members of the Vertebrata. Let us compare, for example, the anterior 

 limb of man with the wing of a bird. Here we have two organs between which 

 the ordinary observer would fail to recognize any resemblance — organs, too, whose 

 functions are entirely different, one being formed for prehension, and the other for 

 flight. When, however, they are compared in the light which a philosophic ana- 

 tomy is capable of throwing on them, we find between the two a parallelism which 

 points to one fundamental type on which they are both constructed. 



There is, first, the shoulder-girdle, or system of bones by which in each case the 

 limb is connected with the rest of the skeleton. Now this part of the skeleton in 

 man is very different in form from the same part in the bird ; and yet a comparison 



