xvi THE ANIMAL KINGDOM. 



to be due to — 1, parasitism ; 2, fixity or immobility of tbe animal ; 3, vegetative 

 nuti'ition, aud 4, excessive reduction in the size of tbe body. 



Analogy and Homology. 



When we comjjare tbe wing of an insect with that of a bird, and see that they are 

 put to tbe same use, we say that tbey are analogous ; for when we carefully comj)are 

 tbe two organs, we see how unlike throughout they are. When we compare tbe fin 

 of a whale with tbe fore-leg of a dog or bear, we see that one is adajjted for swimming, 

 and tbe other for running oA dry land ; but, however unlike tbe two limbs are superfi- 

 cially, we find, on dissection, that all tbe principal bones and muscles, nerves and blood- 

 vessels of tbe one correspond to those in the other, so that we say there is a structu- 

 ral resemblance between tbe two kinds of limbs. Thus analogy implies a dissimilarity 

 of structure in two organs, with identity in use, while homology implies blood-relation- 

 sbip. Analogy repudiates any common origin of the organs, however j)hysiologically 

 alike. 



In tbe early days of zoological science, but little was said about homologies ; but 

 when comparative anatomy engaged the attention of philosojihical students, attention 

 was given to tracing tbe resemblances between organs superficially and functionally 

 unlike. It was found that tbe world of life teemed with examples of homologous 

 parts. 



Afterwards, when the theory of evolution became tbe most useful tool tbe compar- 

 ative anatomist could wield, and when tbe knowledge of comparative embryology 

 completed bis equipment, the most unexpected homologies were discovered. Of 

 course, tbe more nearly related are tbe two animals possessing homologous organs, 

 as tbe dog and whale, the closer and more plainly homologous are their fore limbs. 

 It is easy to trace tbe homologous organs in animals of the same order or class, 

 however effectually degenei-ation on tbe one hand, or differentiation on the other, 

 have done their work. 



But it was then found that tbe branchial sacs of ascidians are homologous with 

 the j)baryngeal chamber of the lamprey eel ; that tbe jiosition of tbe nervous system 

 in ascidians accords morphologically with that of fishes and higher vertebrates ; that 

 the notocord of larval ascidians is the homologue of that of the lancelet and lam- 

 prey, as well as that of embryo vertebrates in general ; and, finally, tbe homologies 

 between the larval ascidians and vertebrates are so startling that many comparative 

 anatomists now maintain that the ascidians belong, with tbe vertebrates, to a common 

 branch of the animal kingdom called Chordata. On tbe other hand, excellent anato- 

 mists ti'ace homologies between certain organs in worms, and corresponding organs 

 in sharks and other vertebrates ; tbe segmental organs of worms have their homologous 

 parts in tbe urogenital organs of sharks ; tbe worm JSaktnof/lossiis has a res])iratory 

 chamber homologous with that of the lancelet and lamprey. Hence it came to pass 

 tb.at these general homologies between the lower, less specialized classes of inverte- 

 brates, particularly tbe worms, and the lower vertebrates, were so many proofs of tbe 

 origin of the latter from worms or worm-like forms. Hence tbe opinion now preva- 

 lent that a homology between organs, however unlike in tbe uses at present made of 

 them, implies that tbe animals having such organs bad a common ancestry. Hence, 

 also, the proofs of the unity of organization of the animal kingdom are based on a 

 profotmd study of tbe resemblances in the tissues and organs of animals, rather than 

 of their superficial, recently-acquired differences. 



