NECESSARY EFFICIENCY OF NATURAL SELECTION. 169 
founded, may be perceived even from the outlines of the doc- 
trine of selection which have just been discussed. Darwin 
assumes no kind of unknown forces of nature, nor hypothetical 
conditions, as theacting causes for the transformation of organic 
forms, but solely and simply the universally recognized vital 
activities of all organisms, which we term Inheritance and 
Adaptation. Every naturalist acquainted with physiology 
knows that these two phenomena are directly connected 
with the functions of propagation and nutrition, and, like all 
other phenomena of life, are purely mechanical processes of 
nature, that is, they depend upon the molecular phenomena 
of motion in organic matter. That the interaction of these 
two functions effect a continual, slow transmutation of or- 
ganic forms, is a necessary result of the struggle for exist- 
ence. But this, again, is no more a hypothetical relation, nor 
one requiring a proof, than is the interaction of Inheritance 
and Adaptation. The struggle for life is a mathematical 
necessity, arising from the disproportion between the limited 
number of places in nature’s household, and the excessive 
number of organic germs. The origin of new species is 
moreover greatly favoured by the active or passive migra- 
tions of animals and plants, which takes place everywhere 
and at all times, without being, however, entitled to rank 
as necessary agents in the process of natural selection. 
The origin of new species by natural selection, or, what 
is the same thing, by the interaction of Inheritance and 
Adaptation in the struggle for life, is therefore a mathe- 
matical necessity of nature which needs no further proof. 
Whoever, in spite of the present state of our knowledge, 
still seeks for proofs for the Theory of Selection, only 
shows that he either does not thoroughly understand the 
