HITCHCOCK: STUDY OF THE LOCAL FLORA 259 



descent are not known to us. We are able to observe, not the 

 complete connecting lines as represented by an infinite number 

 of individuals, but only a cross section of the lines as represented 

 by the organisms of the present. The task of the taxonomist is 

 to form an opinion as to the probable relationships of the in- 

 dividuals. This taxonomic judgment is based upon and is 

 developed by experience. In many cases the evolution has 

 proceeded so far that a group of individuals has become dis- 

 tinctly separated genetically from its allies. The individuals 

 of this group interbreed freely and form a coherent aggregation 

 to which biologists have applied the term species. If all organ- 

 isms had, through the process of evolution and the elimination 

 of intermediates, become definitely segregated into these dis- 

 tinct coherent groups, the task of the taxonomist would be 

 greatly simplified. The species being definite, he would be 

 obliged only to offer guesses or opinions as to the relations of the 

 species, and to classify them into groups. However, evolution 

 is still going on, new species are forming, and old species are 

 becoming extinct. We see, as it were, a still scene from a series 

 of moving pictures representing the development of organisms. 

 In complex group's such as Aster the lines of descent are still 

 nearly parallel. They are clustered, but only here and there 

 is the divergence sufficient distinctly to separate the clusters. 

 More often the clusters are contiguous and can be only arti- 

 ficially separated where the density of the lines is least. 



Thus it is clear that there are two steps in the study of clas- 

 sification. First the facts must be established as to the relative 

 position of these lines of descent. In effect one must plot out 

 in his imagination a cross-section of these genetic lines, the 

 points representing the individuals examined, and the position 

 of the points representing the variation of the individuals from 

 the average of the group in the totality of their morphological 

 characters. The second step is the interpretation of the results 

 in terms of species. In the genus Aster, to which we are 

 constantly returning for an illustrative example, there are 

 certain pencils of lines that stand out distinctly in the plot. 

 There is no difficulty in determining the definiteness of these 



