The Interpretation of Structure. 13 



conspicuous in comparison with those of primitive types, the 

 majority of their differences are such as have resulted from adaptive 

 modifications of structure, by which they have become differently 

 adjusted to the particular conditions of their accepted habitats. 

 Adaptation is one great factor in modifying animal form, produc- 

 ing first divergences, as between one type and its contemporaries; 

 although such features may after-wards become settled in particular 

 groups, and "thus appear for these as primitive, general, or 

 group-characters. Adaptation, in other words, is not a matter 

 of present conditions only, of fixed environment, or an environment 

 of a general or special kind. The rabbit as a gnawing animal or 

 rodent, for example, is also an air-breathing, walking vertebrate, 

 and shares these larger and also more ancient features with many 

 other vertebrates of otherwise 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 primitive 

 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 not 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 retro- 

 gressive 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. 



