CREATION BY EVOLUTION
aquatic plants, became readapted to an aquatic existence. One group after another thus became dominant and then waned or became entirely extinct. The accompanying diagram (Fig. 1) illustrates the successive numerical dominance of different plant types and the increasing complexity of the vegetable kingdom as a whole.
Another principle is illustrated by the progressive loss of plasticity in organisms or organs as they became more complex and more highly specialized. The simpler organisms outlasted the complex or gave origin to new types, for the more complex lost adaptability to new conditions and perished during changes of environment. Most of the earlier forms of the successive groups of plants were synthetic or generalized in structure. The earliest ferns, for example, show combinations of features that subsequently became the property of different fern families, and the seed ferns combined the features of ferns and cycads.
The first simple plants, which grew in the water, probably lacked the substance commonly called leaf green (chlorophyll); they obtained their nitrogen from ammonia compounds and gained their energy by oxidation, in much the same way that some modern bacteria oxidize iron and sulphur. With the development of chlorophyll they were able to utilize directly the carbon dioxide of the air and build up complex organic compounds. The acquisition of this power of using inorganic material for food and of converting sunshine into energy marks the first progressive step in the history of plants. The second step was the occupation of the land. During the long history of land floras, covering millions of years, the two principal advances were the development of what is called secondary wood, such as forms the seasonal layers of the oaks or the pines, which enables them
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