NEW: SPECIES NECESSARILY ARISE, 355 
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 enu- 
merated, 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 estab- 
lished 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 nutri- 
tion 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 numeri- 
cal excess of their germs. But as Adaptation and Inherit- 
ance in the struggle for life are in continual interaction, 
it inevitably follows that natural selection, which every- 
where influences and continually changes organic species, 
must, by making use of divergence of character, pro- 
duce 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 organie 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. 
