COMPARATIVE ANATOMY 



INTRODUCTION. 



I. ON THE MEANING AND SCOPE OF COMPARATIVE ANATOMY. 



A KNOWLEDGE of the natural relationships and ancestral history 

 of animals can only be gained by a comparative study of their 

 parts (Comparative Anatomy) and of their mode of develop- 

 ment (Embryology or Ontogeny). In addition to existing 

 animals, fossil forms must also be taken into consideration (Pa- 

 laeontology), and by combining the results obtained under these 

 three heads, it is possible to make an attempt to trace out the 

 development of the various races or groups in time (Phylogeny). 

 As the different phases of development of the race may be repeated 

 to a greater or less extent in those of the individual, the depart- 

 ments of Phylogeny and Ontogeny help to complete one another. 



It must, however, be borne in mind that in many cases the 

 phases of development are not repeated accurately in the individual 

 that is, are not palingenetic, but that l ' falsifications "of the re- 

 cord, acquired by adaptation, very commonly occur along with 

 them, resulting in ccenogenetic modifications in which the original 

 relations are either no longer to be recognised at all, or are more 

 or less obscured. In this connection, two important factors must 

 be taken into consideration, viz., heredity and variability. The 

 former is conservative, and tends to the retention of ancestral 

 characters, while the latter, under the influence of change in 

 external conditions, results in modifications of structure which are 

 not fixed and unalterable, but are in a state of constant change. 

 The resulting adaptations so far as they are useful to the 

 organism concerned, are transmitted to future generations, and 

 thus in the course of time gradually lead to still further modifica- 

 tions. Thus heredity and adaptation are parallel factors, and a 

 conception of the full meaning of this fact helps us not only to 

 gain an insight into the blood-relationships of animals in general, 

 but also to understand the meaning of numerous degenerated and 



