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Variability is the law of life, and as no two faces are the same, so no two bodies are alike, and no two individuals react alike and behave alike under the abnormal conditions which we know as disease.

In a broad systems sense, an organism's environment is indistinguishable from the organism itself.

The existence of common features in different forms of life indicates some relationship between the different organisms, and according to the concept of evolution, these relations stem from the circumstance that the higher organisms, in the course of millions of years, have gradually evolved from simpler ones.

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.

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.

We have spoken of beings so low in the scale that the individuals throughout their whole existence are not sufficiently specialized to be distinctively plant or animal: yet these are definite life in simpler shape.

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

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.

I naively thought that we could have a molecular definition for life, come up with a set of genes that would minimally define life. Nature just refuses to be so easily quantified.

On the ordinary view of each species having been independently created, we gain no scientific explanation.