The theory which, through Darwin, has been placed at the head of all our knowledge of nature, is usually called the Doctrine of Filiation, or the Theory of Descent. Others term it the Transmutation Theory. Both designations are correct. For this doctrine affirms, that _all organisms_ (viz., all species of animals, all species of plants, which have ever existed or still exist on the earth) _are derived from one single, or from a few simple original forms, and that they have developed themselves from these in the natural course of a gradual change_. Although this theory of development had already been brought forward and defended by several great naturalists, and especially by Lamarck and Goethe, in the beginning of our century, still it was through Darwin, thirteen years ago, that it received its complete demonstration and causal foundation; and this is the reason why now it is commonly and exclusively (though not quite correctly) designated as _Darwin's Theory_.
The great and really inestimable value of the Theory of Descent appears in a different light, accordingly as we merely consider its more immediate connection with organic natural science, or its larger influence upon the whole range of man's knowledge of the universe. Organic natural science, or Biology, which as Zoology treats of animals, as Botany of plants, is completely reformed and founded anew by the Theory of Descent. For by this theory we are made acquainted with the active causes of organic forms, while up to the present time Zoology and Botany have simply been occupied with the facts of these forms. We may therefore also term the theory of descent a _mechanical explanation of organic forms_, or the science of the true causes of Organic Nature.
As I cannot take for granted that my readers are all familiar with the terms "organic and inorganic nature," and as the contrast of both these natural bodies will, in future, occupy much of our attention, I must say a few words in explanation of them. We designate as _Organisms_, or _Organic bodies_, all _living creatures_ or _animated bodies_; therefore all plants and animals, man included; for in them we can almost always prove a combination of various parts (instruments or organs) which work together for the purpose of producing the phenomena of life. Such a combination we do not find in _Anorgana_, or inorganic natural bodies--the so-called dead or _inanimate bodies_, such as minerals or stones, water, the atmospheric air, etc. Organisms always contain albuminous combinations of carbon in a semi-fluid condition of aggregation, which are always wanting in the Anorgana. Upon this important distinction rests the division of all natural history into two great and principal parts--_Biology_, or the science of Organisms (Zoology and Botany), and _Anorganology_, or the science of Anorgana (Mineralogy, Geology, Meteorology, etc.).