THE INTERPRETATION OF STRUCTURE. 12 
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 afterwards 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. 
