THE MEANING OF EVOLUTION 251 



originally simple may undergo progressive modifica- 

 tion : thus the eye of a mollusc may be a simple 

 integumentary cavity in the floor of which there are 

 some simple nerve-endings, and some black pigment ; 

 or this cavity may close up so as to form a sac, and the 

 anterior part of the sac may become transparent so as 

 to form a cornea. Behind the cornea a lens may be 

 formed, and the simple terminal twigs of the nerve- 

 endings may become a many-layered retina of great 

 complexity of structure. In the lowest Chordates the 

 central part of the blood-vascular system is a simple 

 contractile vessel, but this becomes the two-chambered 

 heart of the fish, the three-chambered heart of the 

 reptile, or the powerful four-chambered heart of the 

 warm-blooded animal. Anatomical elements may 

 change in function ; thus parts of the visceral skeleton 

 in the fish may become the ossicles of the middle ear 

 in the Reptiles and Mammals ; while its swim-bladder 

 may possibly be represented in the higher vertebrates 

 by the lungs. 



Thus there may be suppression of parts leading to 

 entire disappearance or to mere vestiges of the original 

 morphology. A structure degenerating through disuse 

 may become removed from its typical relations with 

 other structures and may acquire altogether new ones. 

 Or its increasing importance may lead to its hyper- 

 trophy and to increased complexity of structure, and 

 perhaps to the inclusion of new anatomical elements, 

 or to the incorporation of other parts, the function of 

 which may originally have been quite different. In all 

 sorts of ways organs and organ-systems may become 

 anatomically different as the result of adaptive modi- 

 fications, or indirectly as non-adaptive modifications 

 induced by the adaptive modifications of adjacent 

 parts. It is the task of comparative anatomy to 



