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Evolution is a process of constant branching and expansion.

Evolutionary biologists are not content merely to explain how variation occurs within limits, however. They aspire to answer a much broader question-which is how complex organisms like birds, and flowers, and human beings came into existence in the first place.

We must consider the distinctive characters and the general nature of plants from the point of view of their morphology, their behavior under external conditions, their mode of generation, and the whole course of their life.

They are unique in the simplicity, the diversity, the related harmony of the forms of life that they enclose.

Is it philosophical, is it quite allowable, to assume without evidence from fossil plants that the family or any of the genera was once larger and wide spread? and occupied a continuous area?

Every paleontologist knows that most new species, genera, and families, and that nearly all categories above the level of family appear in the record suddenly and are not led up to by known, gradual, completely continuous transitional sequences.

The way to understand how different species evolved is to think about the niches that they fill in an ecosystem - basically, how they make a living.

Darwin based his theory on generalizations that were strictly empirical. You can go out and see that organisms do vary, that variations are inherited, and that every organism is capable of increasing its numbers in sufficiently favorable circumstances.

As a consequence, geneticists described evolution simply as a change in gene frequencies in populations, totally ignoring the fact that evolution consists of the two simultaneous but quite separate phenomena of adaptation and diversification.

As in all of biology, comparative studies showing differences among species are often helpful for a better understanding of the basic mechanisms; with all its advantages, there is a danger of clinging exclusively to one model organism.