PRINCIPLES OF TAXONOMY 257 



the steps in the evolution of their ancestors. That is, the biogenetic 

 law is less generally applicable than it was formerly supposed to be. 

 However, many important facts of evolution, of limited scope, have been 

 discovered by an appeal to this law. A case in which the recapitulation 

 theory is presumably correct is in the development of gill pouches in all 

 the vertebrate animals. Gills are never developed in the reptiles, birds, 

 and mammals ; but gill pouches are formed in the embryo, and these may 

 actually open temporarily to the outside as gill clefts, between which are 

 the gill bars upon which gills are developed in fishes and amphibia. The 

 production of gill pouches and bars in the higher vertebrates as well as in 

 the lower, besides indicating a common ancestry of all these animals, 

 points to the conclusion that the ancestor was an aquatic animal that 

 respired by means of gills. 



Practical Taxonomy. — The foregoing scheme of genetic classification 

 is a goal toward which taxonomists in general strive. Application of it 

 is attended with some difficulties. One obstacle is that before a satisfac- 

 tory classification of even a small group can be made the species in it 

 must be known. Judgment of kinships rests largely on a comparison of 

 structures, and the characters of each species have an influence on one's 

 judgment of the relationship among other species. Omission of some 

 species tends to modify judgments concerning the whole group. Since 

 there are usually many species in a family, or even a genus, the task of 

 discovering and describing them is no small one. This work has been 

 going on a long time, yet many species are still unknown. Every year 

 many new species are described — few in the groups of large, conspicuous 

 animals, but many in those less generally observed. Because of this 

 still waiting task of describing species, many taxonomists, particularly 

 in the past, have devoted their energies chiefly to naming and putting on 

 record the newly discovered forms. They have had to concern them- 

 selves with kinship to the extent of putting species in the right genera, 

 etc., but they have conceived their main task to be filling out the record. 

 More and more, however, the genetic classification will have to be their 

 aim. 



The large number of species in existence is also a difficulty. Among 

 well over a million, possibly over two million, species no one person can 

 be expert on any considerable fraction. Each taxonomist must limit 

 himself to one group, perhaps an order, often only a family. Names are 

 given to these specialists according to the phyla or classes in which they 

 have competence. An entomologist deals with insects, though he is never 

 an expert in all the orders ; a protozoologist studies the unicellular animals ; 

 an ornithologist knows birds, a herpetologist reptiles or amphibia or both, 

 a mammalogist mammals, etc. 



The other difficulties are mostly those which inhere in the animals 



