COMPAKATIYE ANATOMY 



INTEODUCTION. 



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 Ontogeny and Phylogeny 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 palingeneiic, — but that "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 the eajjobility of 

 variation. 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 

 modifications. 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 gene- 

 ral, but also to understand the meaning of numerous degenerated 



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