I^^TRODUCTION. XVll 



The several organs of which animal forms are 

 made up are all mutually related, or, in other 

 words, certain leading peculiarities of structure 

 are invariably found to co-exist with one another. 

 This is termed the " correlation of forms." Some 

 naturalists, not content with recognising the fact 

 of such co-existence, have forsaken the true scien- 

 tific method, and have sought to discover a causal 

 relationship between the organs thus associated 

 together. But morphology furnishes no answer 

 to these futile inquiries.'* 



"VVe compare the organs of different animals 

 with a view to ascertain whether they agree in 

 structm'e with one another. When such is the 

 case, the corresponding organs are said to be 

 *' homologous." By some the word "morphology" 

 is employed in a restricted sense, to signify the 

 study of homologous organs. And it may here 

 be mentioned that the term Histology has been 

 applied to that branch of morphological science 

 which is specially connected with the investiga- 

 tion of minute structure. 



When two organs perform the same function, 

 they are said to be " analogous " to one anothor. 

 It is necessary to draw an important distinction 

 between analogy and homology. For it is pos- 

 sible that two organs may correspond with one 

 another in both structure and function, or in 

 structure only and not in function, or again in 

 function only and not in structure. 



a 



