We have used very neutral and somewhat figurative words, in order to show what might be called the logical value of the theory of descent, in order to signify its value with respect to so-called “explanation.” We have spoken of the “light” or the “elucidation” which it brings, of the “peculiarity of aspect” which is destroyed by it. We have used this terminology intentionally, for it is very important to understand that a specific though hidden addition is made almost unconsciously to the mere statement of the hypothesis of descent as such, whenever this hypothesis is advocated in order to bring light or elucidation into any field of systematic facts. And this additional hypothesis indeed must be made from the very beginning, quite irrespective of the more detailed problems of the law of transformism, in order that *any* sort of so-called explanation by means of the theory of descent may be possible at all. Whenever the theory that, in spite of their diversities, the organisms are related by blood, is to be really useful for explanation, it must necessarily be assumed in every case that the steps of change, which have led the specific form *A* to become the specific form *B*, have been such as only to change *in part* that original form *A*. That is to say: the similarities between *A* and *B* must never have become overshadowed by their diversities. Only on this assumption, which indeed is a newly formed additional subsidiary hypothesis, joined to the original hypothesis of descent in general--a hypothesis regarding the very nature of transformism--only on this almost hidden assumption is it possible to speak of any sort of “explanation” which might be offered by the theory of transformism to the facts of geography, geology, and biological systematics. Later on we shall study more deeply the logical nature of this “explanation”; at present it must be enough to understand this term in its quasi-popular meaning. What is explained by the hypothesis of descent--including the additional hypothesis, that there always is a prevalence of the similarities during transformism--is the fact that in palaeontology, in the groups of island and continent faunae and florae taken as a whole, as well as in the single categories of the system, the similarities exceed the diversities. The *similarities* now are “explained”; that is to say, they are understood as resting on but one principle: the similarities are understood as being due to inheritance;[144] and now we have but one problem instead of an indefinite number. For this reason Wigand granted that the theory of descent affords what he calls a numerical reduction of problems.