SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION 119 



by balancing the aggregate of their structural characters. In large 

 series, the average of development, not the extremes either way, is 

 taken into account ; so that the lowest members of a higher group 

 may be below the highest members of the next lower group. The 

 common phrase, " below par," or " above par," is most applicable 

 to such cases. 



Machinery of Classifleation. — The inexperienced student may 

 be glad to be given some explanation of the way in which the 

 taxonomic principles we have discussed are applied, and carried into 

 practical effect in classifying birds. Our machinery for that purpose 

 is our inheritance from those naturalists who held very different 

 views from those which touch the evolutionary key-note of modern 

 classification. It is clumsy, and does not work well as a means of 

 expressing the relations we now believe to be sustained by all organ- 

 isms toward one another ; but it is the best we have. Systematic 

 zoology, or the practice of classification, has failed to keep pace with 

 the principles of the science ; we are greatly in need of some new 

 and sharper " tools of thought," which shall do for zoology what the 

 system of symbols and formulae has done for chemistry. We want 

 some symbolic formulation of our knowledge. The invention of a prac- 

 ticable scheme of classification and nomenclature, which should 

 enable us to formulate what we mean by Turdus viscivorus, as a 

 chemist symbolises by SO^Hj what he understands hydrated sul- 

 phuric acid to be, would be an inestimable boon to working 

 naturalists. The mapping out of groups with connecting lines to 

 indicate their genetic relations, in the form of the phylum, is a 

 common practice ; but that, like any other pictorial representation 

 of a " family tree," is not the graphic symbolisation required. The 

 first steps in this direction have been tentatively taken already by 

 the late Mr. A. H. Garrod and others : we already have a mother 

 of the required invention in the necessity of the case, and may hope 

 that the father will not be long in coming. 



Under the present system. Birds are called a " Class " of Verte- 

 brates, and are subdivided into "orders," "families," "genera," 

 " species," and " varieties," as already suflBciently indicated. Groups 

 intermediate to any of these may be recognised ; and if so, are 

 usually distinguished by the prefix sub-. Many other terms are 

 in occasional use, as "tribe," "race," "series," "cohort," "super- 

 family " ; but the six first mentioned are the best established ones 

 among English-speaking naturalists. Their sequence is fixed, as 

 above, from higher to lower, in relative rank.^ With the exceptions 



^ The expression "higher group," in the sense of relative rank in the taxonomic 

 scale, will of course he distinguished from the same expression when applied to the 

 relative rank in the scale of organisation of the objects classiiied. An order of birds 

 is a "higher group" than a family of birds, in the former sense, but no higher than 

 an order of worms, in the latter sense. 



