PLANT AND ANIMAL PROGRESSION 47 



tion of these land and air resources was a forward step in 

 progress. In a relatively short time the plant evolved through 

 a moss-like stage to ferns. Mosses have no true leaves or 

 roots and no conducting tubes (vascular tissue). In the ferns 

 nature completed the invention of the leaf, that marvelous 

 photosynthesis or sugar laboratory, sent roots down into 

 the soil in search of water and minerals, and set up conduct- 

 ing tissue in a two-way traffic from root to leaf. At this 

 time, some 600,000,000 or more years ago, there were vast, 

 shallow, inland seas and swamps, and the ferns spread in an 

 explosively rapid adaptive radiation. Like the moss, the fern 

 reproduced by spores and by union of sperm and egg. In 

 both the moss and fern the sperm had to swim through 

 water to reach the tgg; hence they were and still are con- 

 fined to moist places for the sexual stages of their reproduc- 

 tion. This restriction kept plants away from the drier up- 

 land regions of the continental land masses. 



Today our world presents a panorama of grasses and 

 flowering plants and trees; they were made possible only 

 after nature, in what would seem to be very definitely a 

 progressive step, devised means by which dry sperm (pol- 

 len) could meet the dry egg in the ovary of a flower. How 

 exceedingly difficult this step was, is shown by the record 

 of the rocks and by intermediate forms still living. For 

 several hundred million years nature made tentative efforts, 

 most of them failures, to reach the upland regions; and, with 

 final success, the evolution of the plant was complete, ex- 

 cept for variations of detail. This occurred in the Mesozoic 

 or middle life era, some 150,000,000 years ago. On the basis 

 of an increasing efficiency in the means of exploiting chem- 

 ical and energy sources as fully as possible, the history of 

 the evolution of plants has been progressive. 



In animals the progression has been, in the over-all sense, 

 a truly magnificent parade of advancing forms— in each age 

 new organisms better equipped than previous ones for the 

 exploitation, enjoyment, and understanding of their habitat. 



