. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . me species. There is no absolute test,other than the common judgment of men competent to recognize certain formal rules as assisting in such adecision. A series of fully intergrading- forms, however variedat the extremes, is usually regarded as forming a single are certain recognized effects of climate, of climatic iso-lation, and of the isolation of domestication. These do notusually make it necessary to regard as distinc
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . me species. There is no absolute test,other than the common judgment of men competent to recognize certain formal rules as assisting in such adecision. A series of fully intergrading- forms, however variedat the extremes, is usually regarded as forming a single are certain recognized effects of climate, of climatic iso-lation, and of the isolation of domestication. These do notusually make it necessary to regard as distinct species theextreme forms of a series concerned. In the words of the entomologist Rambur, A species is agroup of beings which in successive generations show the samecharacters of organization, unchanged so long as the localityand external conditions remain unchanged. 14 EVOLUTION AND ANIMAL LIFE The number of species actually existing is far beyond ordi-nary conception. The earliest serious attempt to catalogue thespecies of animals and plants was made by Linnaeus. In thetenth edition of his Systema Nature in 1758, in the 823 pages f. L_FIG. 3.—Brittle or serpent stars—species undetermined. (Natural size.) devoted to animals, he describes and names some four thousanddifferent kinds. Great as this number seemed, Linnaeus ven-tured to suggest that probably his pages did not include half ofthose kinds of animals actually existing. To-day our records contain descriptions of more than onehundred and fifty times as many kinds of animals as were knownto Linnaeus and all his predecessors and all his associates of acentury and a half ago. Each year, since 1864, there has beenpublished in London a volume called the Zoological Record/Each of the volumes—larger than the whole Systema Naturae-contains the names of the animals new to science which havebeen added to the system in the year of which it treats. In the VARIETY AND UNITY IX LIFE 15 record of each year we find about twelve t
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