Aim and Method of Comparative Analomy 373 



Form and structure must be carefully considered. In general, 

 homologous organs are similar in form and structure. But it must never 

 be forgotten that form and function are vitally related. Organs which 

 are not homologous may perform similar functions and, accordingly, 

 may have similar form and structure. The cuttlefish (Sepia, a ceph- 

 alopod mollusk) has an eye whose very complex structure is closely 

 similar to that of a vertebrate eye, but there can be no genetic relation 

 between the two. External form is much less significant than internal 

 structure. In external form, the horse's front leg, human arm, bird's 

 wing and whale's flipper are very different, but the internal structure of 

 all of them is, with adaptive modifications, that of a typical pectoral 

 appendage. Livers are highly variable in form. Lungs range from the 

 simple slender tubular sac of tailed amphibians (urodeles) to the mas- 

 sive several-lobed mammalian lung minutely subdivided internally. 

 Brains, by contrast, are more stable in form. All vertebrate brains show 

 externally five main divisions. Differences in the form of the brains of 

 various vertebrates consist mainly in differences in the relative bulk of 

 the several regions. 



In so far as form is independent of function (e.g., as in the gross 

 form of a liver), it is, so to speak, left free to vary according to the in- 

 cidental circumstances in which the organ finds itself. A liver looks as 

 if its form had been impressed upon the organ by impact of neighboring 

 organs. The extreme difference between the form of a feather and that 

 of a hair throws grave doubt upon their homology but, taken alone, is 

 not sufficient to prove that a feather is not a highly elaborated hair. 

 Facts quite apart from the difference in form make it likely that 

 feathers and hair had common origin in horny scales of reptiles and 

 therefore are only remotely homologous. 



Structure implies differentiation of constituent parts and a set of 

 definite and correlated relations of one part to others. All of this, again, 

 hangs upon function. Nevertheless, it is possible that a considerable 

 change in the function of the whole and in the external form of the 

 whole may occur without so radically altering internal structure as to 

 make the organ unrecognizable as the original organ. The flipper of a 

 whale looks like the fin of a fish, but the internal structure of the flipper 

 (Fig. 579) corresponds to that of the foreleg of a land mammal. The 

 flipper as a whole is homologous with the pectoral fin of a fish, but the 

 immediate homologies of the bones constituting the skeleton of the 

 flipper are with bones of the foreleg of a quadruped, the wing of a bird, 

 and the human arm. 



The grade of structure to which the anatomist must give chief 

 attention in his search for homology is that of the organ. The structure 

 of the tissues and cells of which the organ is constituted should not be 



