176 SCIENCE SKETCHES. 



moLis diversity among living things ; and second, 

 their even more surprising unity. The half million 

 known forms of animals and plants may be readily 

 reduced to less than a dozen special forms or types. 

 The problem is to account for the origin of this 

 diversity in life in some way which shall not leave 

 the essential unity out of sight. 



The number of different forms of life on the 

 earth, now recognized as different species, is far 

 beyond the usual conception of those who have 

 not made such matters a special object of study. 

 This old book which I hold in my hand is a copy 

 of the half of the tenth edition of the " Systema 

 Naturae," which treats of all the known species of 

 animals. In its eight hundred and twenty-three 

 pages, some four thousand different kinds of ani- 

 mals are named and briefly described. These four 

 thousand species were all that were known in the 

 world by civilized man a little more than a century 

 ago. But for every one of these enumerated by 

 Linnaeus, more than fifty kinds are known to the 

 naturalists of to-day, and the number of species 

 still unknown doubtless far exceeds the list of those 

 already recorded. Every year, for the last quarter 

 of a century, there has been published in London 

 a plump octavo volume known as the " Zoological 

 Record." Each of these volumes is larger than the 

 whole ** Systema Naturae." Each volume is filled 

 with the names of the animals new to science 

 which have been added to our lists during the year 

 of which it treats. Every one of these volumes 

 contains the names of two or three times as many 

 animals as are mentioned in the whole " Systema 



