116 THE AMERICAN NATURALIST [Vol. XLII 



sive period. Aristotle, who is usually regarded as the 

 father of natural history and who lived in the fourth cen- 

 tury B. C, mentioned in his zoological treatises about 480 

 kinds of animals. Linnaeus, in the tenth edition of his 

 "Systema Naturae," published in 1758, described 4,378 

 species. Giinther estimated that in 1830 a total of 73,588 

 species had been reached and that in 1881 the. number had 

 mounted to 311,653. Sharp placed the total in 1896 at 

 386,000, and, judging from the rate of increase in the 

 vertebrates and echinoderms, the total number of de- 

 scribed species at present must be approximately 500,000. 

 From these estimates, which include only living species, it 

 must be clear that in the twenty-two centuries between 

 Aristotle and Linnaeus the number of species known to the 

 naturalist had increased only ten-fold, while in less than 

 a century and a half after Linnaeus the increase had been 

 over a hundred-fold. 



This enormous increase has been the result of two proc- 

 esses: the actual discovery of new forms in nature and 

 the subdivision of what was originally regarded as one 

 species into two or more species. The actual discovery 

 of new forms implies the gradual exhaustion of nature 

 and the conclusion of this process will come when explora- 

 tion can yield no more new species. That the naturalist 

 even of to-day is far from this goal is only too well known 

 and can be illustrated by the following instances. 



All of you doubtless have seen a delicate pearly shell, 

 chambered somewhat like a miniature nautilus, but with 

 its whorls open. This shell, which is known as the 

 Spirula, is found commonly on the shores of the tropical 

 oceans and may even reach our more northern coasts. It 

 is extremely fragile and hence it can not last long on a 

 surf-beaten shore. Nevertheless it is sometimes so abun- 

 dant on tropical beaches as to form veritable windrows. 

 Each shell is the life product of a single Spirula annual, 

 which, so far as one can judge from the abundance of 

 shells, must be a common inhabitant of the tropical seas. 

 And yet, aside from fragments, the Spirula animals thus 



