CHAPTER X. 



OTHER THEORIES OF SPECIES-FORMING AND 

 DESCENT (CONTINUED) : THEORIES ALTER- 

 NATIVE TO SELECTION. 



WE come now to the brief consideration of three general 

 theories, or groups of theories, which are offered more as 

 alternative or stibstitutionary theories for natural selection 

 than as auxiliary or supporting theories. These groups of 

 theories are the Lamarckian one, based on the inheritance 

 of characters acquired individually (ontogenetically) during 

 the lifetime of the organism due to the effects of use and 

 disuse and functional stimuli ; the general conception of 

 orthogenesis variously provided for by Nageli, Eimer, 

 Jaeckel (metakinesis), and others, and finally the theory of 

 heterogenesis, suggested by von Kolliker, definitely formu- 

 lated by Korschinsky, and most recently, and importantly, 

 developed by de Vries. Few biologists would hold any of 

 these theories to be exclusively alternative with natural 

 selection; de Vries himself would restrict natural selection 

 but little in its large and effective control or determination 

 of the general course of descent. But all of these theories 

 offer distinctly substitutional methods of species-forming, 

 and one of them includes certainly the most favoured expla- 

 nation, next to selection, of adaptation, while the authors or 

 later up-holders of some of them actually deny any con- 

 structive, that is, adaptational, species-forming or descent- 

 controlling, influence of natural selection. 



The Lamarckian Theory. It is a great presumption to 

 attempt to offer in so small a book as this any exposition of 

 a theory so long known and elaborately developed as the ex- 



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