IMMUTABILITY OF SPECIES. 397 



that there are certam periods marked by an entire 

 extinction of the species of flora and fauna, and a 

 re-creation under the same types, classes, and orders. 

 As Professor Agassiz remarks, this genesis is so 

 various among all the different branches, taking 

 place at the same time, how could the same physical 

 influences produce such opposite characters 1 The 

 four types of vegetable life are represented as present 

 in the earliest formations ; but each had its epoch 

 of supremacy. The Carboniferous period was dis- 

 tinguished by its remarkable acrogens. The Lipo- 

 dendra, or Club-mosses grew from fifty to sixty 

 feet in height, and their appearance must have been 

 beautiful. The stems of these giants were strength- 

 ened by bands or buttresses of vascular fibre run- 

 ning vertically. The curious Sagillaria was con- 

 temporaneous. The carvings in its trunk resembled 

 designs for some highly ornamental style of archi- 

 tecture; knots single or double, circles, graceful 

 grooves, crescents or eyes, protruded from their 

 trunks at regular distances. Their roots were alike 

 marked by peculiarities, and probably also their fern- 

 like foliage. The Oolitic period brought the beau- 

 tiful and varied Thujas, the Biotas, the Pine, and 

 other relatives of the Conifera. Then the gym- 

 nogens were supreme — then wild forests of ever- 

 greens covered the earth. But in the Tertiary 

 deposits, the plants of the Carbonifera sink to their 

 present size, the Conifera occupy only a portion of 

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