144 Elementary Zoology. 



organs, though they perform similar functions. The organs 

 are, in fact, analogous, just as are (to use the most familiar 

 instance of analogy) the wings of a butterfly and of a bird. 



When organs correspond in structure and development, 

 they are then said to be homologous. Thus there is no 

 doubt that the heart of a man and of a shark are homologous 

 structures. 



But both these terms require further expansion and 

 definition before they can become really useful. In extreme 

 cases there can be no doubt as to which category a resemblance 

 between two sets of organs can be referred. The leaves of 

 a tree are in a sense the equivalents of the stomach and lungs 

 of a man. In both organs are food assimilation and respiration 

 carried on. But no one would assert that there is more than 

 a not very strong analogy. 



On the other hand, we may take such an example as the 

 correspondence or non-correspondence between the heart of an 

 anodon and that of a frog. Now, here we have two organs 

 which play a like part — they are in each case the central organ 

 of impulsion of the vascular system. Furthermore, there is 

 this structural likeness between them, that both are divided 

 into three chambers, of which two receive the blood coming 

 from the veins, while the other, the ventricle, is concerned with 

 the driving of the blood through the arteries. Finally, both 

 organs lie in a pericardium. Yet no competent anatomist will 

 dispute the assertion that these organs are not homologous, 

 in spite of their almost detailed likeness. They are only 

 analogous ; but to an analogy of this kind it is useful to apply 

 the term " homoplasy." This term signifies a similar moulding, 

 an adaptation to similar needs. There are even more striking 

 instances of, apparently, similar structures, which are yet 

 different. It is not at all certain, for instance, that the two 

 ventricles of the bird's heart are severally homologous with 

 the two ventricles of a mammal's heart. 



It would seem at first sight absurd to doubt that the four 

 cavities of the heart of the higher vertebrates, mammals, and 

 birds are really and truly homologous. But the doubts that 

 exist are due to the great dissimilarities in other points of 



