A VIE W IN PERSPECTIVE. 121 



A second fact of equal significance is that not only 

 has no large type disappeared, but no new great type 

 of animals has appeared since the Silurian (2) age. 

 This has been sufficiently discussed, and only need 

 to be mentioned here as correlated with the subject 

 in the last paragraph. 



During the geological ages species differed in 

 their range as they do to-day. Some were confined 

 to small localities (certain species of trilobites), while 

 others had an almost world-wide range (Foraminifera). 

 As would be expected, the species with a long range 

 in time were usually the ones that had also a large 

 geographical range (Saccuminina Carteri). To this, 

 however, there are exceptions, for some species con- 

 fined to a single system of rocks have an enormous 

 range in geographical area (certain ammonites). 



In looking over the whole geological period we find 

 that after a species of animals has once disappeared 

 it has never again reappeared, no single instance 

 being known which would serve as an exception to 

 this rule. The principle may be carried out still 

 further. No type of animals is ever a second time 

 developed. It is probably even correct to say that 

 throughout the wide animal kingdom no organ that 

 has fully disappeared is ever again redeveloped. 

 Animals cannot develop a second time that which 

 they have once completely lost ; when a change of 

 circumstances requires an animal to perform again a 

 function formerly performed by a lost member, the 

 lost member is not redeveloped, but some other 

 organ assumes a new function. For instance, the 

 insects once possessed many legs but they have uni- 



