THK BEGINNINGS OK COMrAKATIVK. ANATOMY 



Historia Animalium. Generally speaking, parts which bear 

 the same name are for Aristotle homologous throughout 

 the class. But he goes further and notes the essential 

 resemblance underlying the differences of certain parts. He 

 classes together nails and claws, the spines of the hedgehog, 

 and hair, as being homologous structures. He says that 

 teeth are allied to bones, whereas horns are more nearly 

 allied to skin (//is/. Aniin., iii.). This is an astonishingly 

 happy guess, considering that all he had to go upon was the 

 observation that in black animals the horns are black but the 

 teeth white. One cannot but admire the way in which 

 Aristotle fixes upon apparently trivial and commonplace 

 facts, and draws from them far-reaching consequences. He 

 often goes wrong, it is true, but he always errs in the grand 

 manner. 



While Aristotle certainly recognised the existence of 

 homologies, and even had a feeling for them, he did not 

 clearly distinguish homology from analogy. He comes 

 pretty near the distinction in the following passage. After 

 explaining that in animals belonging to the same class the 

 parts are the same, differing only in excess or defect, he says, 

 " But some animals agree with each other in their parts 

 neither in form nor in excess and defect, but have only an 

 analogous likeness, such as a bone bears to a spine, a nail to 

 a hoof, a hand to a crab's claw, the scale of a fish to the 

 feather of a bird, for that which is a feather in the bird is a 

 scale in the fish" (Cressvvell, he. cit., p. 2). One of these 

 comparisons is, however, a homology not an analogy, and the 

 last phrase throws a little doubt upon the whole question, for 

 it is not made clear whether it is position or function that 

 determines what are equivalent organs. 



In the /)< l\irtihns Aiiiinalimn there occurs the following 

 passage: "Groups that only differ in degree, and in the 

 more or less of an identical element that they possess, are 

 aggregated under a single class ; groups whose attributes are 

 not identical but analogous are separated. For instance, 

 bird differs from bird by gradation, or by excess and defect ; 

 some birds have long feathers, others short ones, but all are 

 feathered. Bird and Fish are more remote and only agree in 

 having analogous organs; for what in the bird is feather, 



