PHYLOGENY AND CLASSIFICATION. 87 



tective coloration or structure, mimicry, polymorphism, 

 and other such factors, greatly complicate the problem 

 of the evolution of secondary sexual characters. 



CHAPTER VII 



PHYLOGENY AND CLASSIFICATION 



IT is the task of the anatomist and systematist to study 

 and compare the structure of adult organisms (Compara- 

 tive Anatomy or Morphology), and also their develop- 

 ment (Embryology), in order to make out their affinities, 

 to discover the lines of descent which connect the various 

 diverging branches of the genealogical tree. These 

 branches are called Phyla, and Phylogeny is the name 

 given to the study of the pedigrees of organisms, the 

 tracing out of their blood-relationships. A true natural 

 classification is based on phylogeny. Much valuable 

 evidence for this science can be derived from Palaeon- 

 tology, the study of fossil extinct animals and plants, 

 which may reveal the actual ancestral forms or their 

 near relatives ; but a great deal can be gathered also from 

 a knowledge of the structure and development of the 

 living. 



Since forms which differ widely in the adult condition 

 often resemble each other much more closely in the 

 young or embryonic stages, by observing their develop- 

 ment affinities can sometimes be discovered which would 

 not otherwise be suspected, or at all events would be very 

 doubtful. Familiar instances are those of the Tunicates 

 and Cirripedes. The former are the Sea Squirts, seden- 

 tary animals for the most part, which have undergone 

 degeneration owing to their peculiar mode of life. The 

 adult lives fixed to the sea bottom, and is of very simple 

 structure, showing little resemblance to an ordinary 



