xvi THE ANIMAL KINGDOM. 



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

 nutrition, and 4, excessive reduction in the size of the body. 



AjfAioGY AND Homology. 



When we compare the wing of an insect with that of a bird, and see that they ai-e 

 put to the same use, we say that they are analogous ; for when we carefully compare 

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

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

 and the other for running on dry land ; but, however unlike the two limbs are supei-fi- 

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

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

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

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

 ship. Analogy repudiates any common origin of the organs, however physiologically 

 alike. 



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

 when comparative anatomy engaged the attention of philosophical students, attention 

 was given to tracing the resemblances between organs supei-ficially and functionally 

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

 parts. 



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

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

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

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

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

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

 however effectually degeneration on the one hand, or differentiation on the other, 

 have done their work. 



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

 the pharyngeal chamber of the lamprey eel ; that the position of the 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, the homologies 

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

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

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

 mists trace homologies between certain organs in worms, and corresponding organs 

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

 parts in the urogenital organs of sharks ; the wonn Balanoglossus has a respiratory 

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

 that these general homologies between the lower, less specialized classes of inverte- 

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

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

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

 them, implies that the animals having such organs had a common ancestry. Hence, 

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

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

 of their superficial, recently-acquired differences. 



