ZOOLOGICAL POSITION 5 



series from primitive to specialized types. However, the majority 

 of their differences are such as have resulted from adaptive modifi- 

 cations of structure, by which they have become differently ad- 

 justed to the particular conditions of their accepted habitats. 

 Adaptation to environment is one great result of the modification 

 of animal form, and is revealed in part by structural divergences, 

 as between one type and its contemporaries; although such features 

 may afterwards become settled in particular groups, and thus 

 appear for these as primitive, general, or group characters. Adap- 

 tation, in other words, is not a matter of present conditions only. 

 The rabbit as a gnawing mammal , a lagomorph , for example , is also an 

 air-breathing, walking vertebrate, and shares these relatively large 

 and ancient features with many other vertebrates of different kinds. 



It is customary to include under the term specialization all 

 those features in which an organism may be shown to be more 

 highly modified in comparison with another type. If the latter is 

 an ancestral type, or a lower form exhibiting ancestral features, 

 its more primitive features are said to be prototypal, because they 

 indicate the form from which the higher modification has been 

 derived. Such comparisons not only reveal the fact that different 

 animals are specialized in different degrees, but also show that a 

 given form may be greatly specialized in some respects and primi- 

 tive in others. 



Moreover, it is to be considered that animals are at the present 

 time, as they have been in the past, more or less changeable, or 

 plastic types. Some of the most interesting features which they 

 exhibit depend on the circumstance that the adjustment of structure 

 which is rendered necessary by the opposing effects of heredity and 

 specialization is gradual rather than exact or immediate. Thus, it 

 is not difficult to find in any specialized animal, in addition to those 

 organs which are functional or in full development, others which 

 are retrogressive in character and reduced in size. It is also to be 

 assumed, although difficult of proof among living forms, that there 

 are also organs which are sub-functional or progressive. 



ZOOLOGICAL POSITION 



It will be evident from the foregoing statement that every 

 specialized animal possesses in its organization a vast assemblage 



