204 THE PERPETUATION OF ADAPTATIONS 



biramous structures which, with growth, lose their generalized 

 structure and become specialized. In different regions of the 

 same organism and in different organisms, therefore, the same 

 generalized type may become adapted for quite diverse activities. 

 Thus the fourth pair of thoracic appendages in Mysis (Schizo- 

 pod), Squilla (Stomatopod), and the lobster (Decapod) are, at 

 some period in development, the same in structure, and resemble 

 one another, but in Mysis they remain biramous and lamella- 

 like; in Squilla they become modified into characteristic maxilli- 



FIG. 87.- A schizopod (Gnathophausia gigas) with permanent biramous tho- 

 racic appendages (cf. schizopod stage, in development of the lobster, Fig. 78). 

 (From Sars.) 



peds or food getting organs, and in the lobster they change into 

 the powerful offensive and defensive chelate walking legs char- 

 acteristic of the Decapods. Such differences form the basis of 

 animal species. 



A. ANIMAL DESCENT 



The lesson taught by the structures and functions of the 

 lobster's appendages may be extended to all animals. As the 

 specialized appendages of different Crustacea may be traced 

 back to generalized structures, so may animals, no matter how 

 modified, be traced back, more or less accurately, to more 

 generalized types from which they have descended. This 

 descent is often difficult to make out, for some of the primitive 

 structures may have become so modified, through functional 

 activity or otherwise, as to be unrecognizable; others, like the 



