NEW SPECIES NECESSAEILY 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 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. 



