(10.) _The unity of Biology as a whole_, the deep inner connection existing between all the phenomena named and all the other phenomena belonging to zoology, protistics, and botany, and which are simply and naturally explained by a single common principle. This principle can be no other than the common derivation of all the specifically different organisms from a single, or from several absolutely simple, primary forms like the Monera, which possess no organs. The Theory of Descent, by assuming this common derivation, throws a clear light upon these individual series of phenomena, as well as upon their totality, without which their deeper causal connection would remain completely incomprehensible to us. The opponents of the Theory of Descent can in no way explain any single one of these series of phenomena or their deeper connection with one another. So long as they are unable to do this, _the Theory of Descent remains the one adequate biological theory_. We should, on account of the grand proofs just enumerated, have to adopt Lamarck’s Theory of Descent for the explanation of biological phenomena, even if we did not possess Darwin’s Theory of Selection. The one is so completely and _directly proved_ by the other, and established by mechanical causes, that there remains nothing to be desired. The laws of _Inheritance_ and _Adaptation_ are universally acknowledged _physiological_ facts, the former traceable to _propagation_, the latter to the _nutrition_ of organisms. On the other hand, the _struggle for existence_ is a _biological_ fact, which with mathematical necessity follows from the general disproportion between the average number of organic individuals and the numerical excess of their germs. But as Adaptation and Inheritance in the struggle for life are in continual interaction, it inevitably follows that _natural selection_, which everywhere influences and continually changes organic species, must, by making use of _divergence of character_, produce new species. Its influence is further especially favoured by the active and passive _migrations_ of organisms, which go on everywhere. If we give these circumstances due consideration, the continual and gradual modification or transmutation of organic species will appear as a biological process, which must, according to causal law, of _necessity_ follow from the actual nature of organisms and their mutual correlations.