EVOLUTION AND DISTRIBUTION OF ANIMALS. 313 



EVOLUTION AND THE DISTRIBUTION OF ANIMALS.* 



By DAVID STARR. JORDAN, 



PRESIDENT OF THE UNIVERSITY OF INDIANA. 



I. 



""VTO one with good eyes and brains behind them has ever 

 -i-N looked forth on the varied life of the world — on forest or 

 field or brook or sea — without at least once asking himself this 

 question : " What is the cause of nature's endless variety ? " We 

 see many kinds of beasts and birds and trees and flowers and in- 

 sects and blades of grass, yet when we look closely we find not 

 one grass-blade in the meadow quite like another blade. Not 

 one worm is like its fellow-worm, and not one organism in 

 body or soul is the measure of its neighbor.' You may search 

 all day to match one clover-leaf, and, should you succeed, even 

 then you have failed ; for, if the two leaves agree in all physical 

 respects, they may still be unlike in that which we can not see, 

 their ancestries, their potentialities. Again, with each change of 

 conditions, of temperature, of moisture, of space, of time, with 

 each shifting of environment, the ranga in variety increases. 

 " Dauer in Wechsel " (persistence in change) ; " this phrase of 

 Goethe," says Amiel, "is a summing up of nature." And the 

 naturalist will tell you that the real variety is far greater than 

 that which appears. He will tell you that, where commonness 

 seems to prevail, it is the cover of variety. The green cloak 

 which covers the brown earth is the shelter under which millions 

 of organisms, brown or green, carry on their life-work. 



Each recognizable kind of animal or plant is known in biology 

 as a species. The number of forms now considered as distinct 

 species is far beyond the usual conception of those who have not 

 made a special study of such matters. I have an old book in my 

 library, the tenth edition of the Systema Naturse, published by 

 Linngeus in 1758. This book treats of all the species of animals 

 known a little more than a century ago. In its eight hundred 

 and twenty-three pages some four thousand different kinds of 

 animals are named and briefly described. But for every one of 

 these enumerated by Linnaeus, more than one hundred kinds are 

 known to the modern naturalist, and the number of species still 

 unknown doubtless exceeds the number of those already recorded. 

 Every year for the last quarter of a century there has been pub- 

 lished in London a plump octavo volume known as the Zoological 

 Record. Each of these volumes, larger than the whole Systema 



* An address delivered before the Chicago Institute, in a course on the Testimony of 

 Science in regard to Evolution. 

 vol. xxxvii. — 24- 



