Interprktation of Structure. 5 



nature of which is easilv understood, although the process by which they 

 have been developed is still a matter of uncertainty. In comparison 

 with one another, animals present certain resemblances and differences — 

 diagnostic features, which are used as a basis for classifying them into 

 major and minor groups. In many cases characters of resemblance have 

 been shown to be secondary, and are hence described as convergent. In 

 some of these the resemblances are of a gross type, and the structures are 

 described as analogous; in other cases they are exact or homoplastic. 

 As a rule, however, characters of resemblance are broad marks of affinity, 

 comparable to those seen on a small scale in human families, or in human 

 races, and determined as in the latter cases by heredity. The chief basis 

 of comparison of animals with one another is the general assumption 

 that structures which are similar or identical are homogenous — of com- 

 mon origin. On the other hand, their differences are chiefly marks of diver- 

 gence in evolution. Although it is conceivable that many of the internal 

 features of animals are the result of a general progressive development, 

 more 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 differ- 

 ently adjusted to the particular conditions of their accepted habitats. 

 Adaptation is one great factor in modifying animal form, producing 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 neces- 

 sary 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 retrogressive in character and reduced in size. It is 

 also to be assumed, although difi^cult of proof, except where the inter- 

 mediate member of a known series is being considered, that there are also 

 organs which are sub -functional or progressive. 



