Systematic study of the birds has brought the number of species and subspecies known to inhabit North and Middle America to above 3000. The most comprehensive systematic treatise is the still incomplete report of Ridgeway[[176]] of which seven large volumes have already been issued.
On the reptiles, the most complete monograph is that by Cope[[177]] entitled “The Crocodilians, Lizards and Snakes of North America.”
The Amphibia have also been studied by Cope, whose report on the Batrachia of North America[[178]] is the standard taxonomic work.
The most comprehensive systematic work on fishes is the “Descriptive Catalogue of the Fishes of North and Middle America” by Jordan and Evermann.[[179]]
The invertebrate groups have been in part similarly monographed by the members of the U. S. National Museum staff and others, and further studies are in progress. Other taxonomic monographs published by this museum include the various groups of animals from many different parts of the world.
A number of the larger State, municipal, and university museums publish bulletins on special groups represented in their collections as well as articles of general zoological interest.
Expeditions, subsidized by museum and private funds, are from time to time sent to various parts of the world and their results are often published in sumptuous manner.
The total number of living species of animals is unknown, but considering that about a quarter of a million new species have been described during the past thirty years, it is probable that several million species are in existence to-day. More than half a million have been described. These are probably but a small fraction of the number that have existed in past geological ages.
Thus, in spite of all the work that has been done in systematic zoology and as the number of known species continues to increase, there still remain many groups of animals, some of which are by no means rare or minute, in which probably only a small proportion of the species are as yet capable of identification.
It is only since the publication of Ward and Whipple’s “Fresh-water Biology” within the past year that the amateur zoologist could hope to find even the names of all the organisms which may be collected from a single pool of water. And in many cases he will still meet with disappointment, for many of our protozoa and other fresh-water organisms have not yet been described as species.