4 COMPARATIVE ANATOMY. 



logical duties may thus bring organs which are morphologically 

 connected into very different categories. Thence results the sub- 

 ordinate importance which we must assign to the physiological 

 duties of an organ when we are engaged in an investigation in 

 Comparative Anatomy. Physiological value is only to be regarded 

 at all, and then in the second place only, when we are trying to 

 make out the relations to the entire organism of those modifica- 

 tions which an organ may have undergone as compared with some 

 other condition of the organ. 



By this examination of anatomical facts, by means of com- 

 parison, Comparative Anatomy demonstrates the connection of entire 

 series of organs. Within these series we find changes of the 

 most varied range, sometimes slightly, sometimes widely extended ; 

 modifications which affect the size, number, form, and even the 

 texture, of the parts of an organ, and which may even lead to 

 changes in its situation. The review of such a series teaches us 

 then to recognise a progress presented in those several successive 

 stages, which the modifications of one and the same organ in different 

 animals exhibit to us. 



§ 4. 



We ascribe the existence of a certain amount of similarity in 

 the organisation of certain larger or smaller divisions of the animal 

 kingdom to Transmission — a phsenomenon which is exhibited in 

 the passing on of its organisation by a given organism to its posterity. 

 The descendants repeat the organisation of the parental organisms. 

 This is an indubitable fact. Nevertheless now and again objections 

 are raised either to the existence of Transmission or to its signifi- 

 cance. The similarity of the organisation of the descendants and 

 their ancestors is then ascribed, not to Transmission, but to certain 

 physical forces acting during embryonic life. In reply, we may 

 ask, how does it happen that in ancestor and descendant these 

 forces are the same — viz. all those forces of tension, of pressure, 

 and so on, from which it is sought to deduce the building up of the 

 embryo ? If, for example, a joint gets its ontogenetic development 

 by the movement of the parts of the skeleton by means of muscular 

 activity, a certain arrangement of the muscles is presupposed, and 

 a perfectly definite structure of the muscles; and for these again, 

 a perfectly definite number and arrangement of the morphological 

 elements which make up a muscle. This being so we must ask, 

 whence comes the definite arrangement of these parts ? whence arises 

 the similarity of arrangements in the ancestors and the descendants ? 



We find, in fact, that we must give full recognition to the exist- 



