6 Mr. W. S. Macleay 07i the Comparative Anatomy 



able work, entitled Tabula Affinitatum Animalium, and published 

 in 1783, may have intended to keep some such principle as this 

 in view : but as with him, unfortunately, the slightest analogy 

 constituted an affinity, we may understand how he found it 

 impossible to trace the mode in which structures vary, and 

 much more so to apply the maxim of variation to arrangement. 

 On a cursory glance at the principles of arrangement laid down 

 by Aristotle* at the commencement of his Histojia Animalium, 



he 



* It can scarcely be doubted that Aristotle Would have followed this principle, as 

 well as have made the proper distinction between affinity and analogy, if he had looked 

 less to the differences of particular organs and more to the affinities of general struc- 

 tures. Indeed he appears to have had a glimpse of the two great principles of natural 

 arrangement, and was only ignorant of the proper mode of using them. His views of 

 the subject are really .curious when compared with our modern notions of zoology. 

 The parts of animals, he says, either agree with or differ from each other in four prin- 

 cipal ways. — Now here, at the opening, lies the grand cause of his not thoroughly un- 

 derstanding the matter : for if he had said, that Animals themselves, instead of their 

 organs, may be arranged by four methods, it will be manifest, from the enumeration of 

 his four methods, that he could not have failed to arrive at the truth. 



1. Organs, he says, may be arranged, first, according to the natural groups {xcura to 

 yevos, or xut siSoj), which, as for instance Birds or Fishes, depend on a similar construc- 

 tion of parts. That relation, he proceeds to state, which the whole bears to the whole, 

 the group being the same, the part must bear to the part. Now this is an axiom 

 which, however true with respect to quantity, will not hold good with respect to struc- 

 ture : for were it true, it would follow that, in the natural group of jdves, for instance, 

 a frugivorous bird could not have the same form of beak as a bird of prey ; whereas we 

 know the contrary. The fact is, that if Aristotle had said that animals and not their 

 parts are to be arranged according to their natural groups, he would have expressed 

 the great principle of natural affinity : but a mathematical axiom made him unluckily 

 think, that the classification of organs was the same thing with the classification of 

 the animals to which they belong. 



2. Secondly, he says. Organs may be arranged according to their excess and defect. 

 (xa9' uTrepo^yiv xati sXAenf/iv). This being entirely a consideration of quantity, and not of 

 form, his mathematical axiom comes into play. His opinion is accordingly correct, 

 that animals are capable of a binary distribution, depending entirely on the excess or 

 defect of particular organs ; as where he instances birds being divisible into those with 



long 



