CHAPTER III 



Pliylogeny. 



. §1. Natural Classification and Phyloqeny. 



Definition. — Since the idea of the evolution of species is no 

 longer a simple conjecture, but is based on certain scientific data, 

 the investigation of genealogic trees of living organisms or of 

 fossil forms has acquired considerable importance, and there is 

 scarcely any systematic work either on Zoology or Palaeontology, 

 which does not conclude with a more or less extended essay in 

 that direction. D.irwin contented himself with establishing 

 on a solid basis the principles of the doctrine of transformation, 

 and left to his successors the task of deducing the consequences. 

 He demonstrated that species are derived one from the 

 other, and that consequently there exist between all organisms, 

 both living and extinct, veritable relations of parentage more or 

 less removed. Phylogeny is the determination of these ties ; it is 

 the investigation of the descent, not only of allied species, but 

 a.lso of the most extensive groups, in fine of every form, both of 

 the animal and vegetable kingdoms. 



Principles of classification. — The problem of Phylogeny is 

 onl}' a new form, due to new ideas, of the problem of natural 

 classification, which has confronted us from the time when living 

 organisms first began to be objects of serious study. Buffon 

 opposed every idea of classification, while Linne, the first who 

 established a substantial classification, considered the taxonomic 

 method simply a convenience for abridging the exposition of 

 characters and for facilitating researches, l^evertheless toward 

 the close of his life he indicated the path to be followed in order 

 to arrive at a more rational principle. Jussieu was the first to 

 establish in an authoritative manner the principles of a natural 

 classification. He devoted his energies to presenting as faithfully 

 as possible a demonstration of the relations which exist among 

 all the types of the vegetable kingdom. In order to realize this 



