446 EVOLUTION, GENETICS, AND EUGENICS 



Lamarckian hypothesis claun that mutations, as shown in the experi- 

 ments so assiduously conducted by the Mendelians, are valueless as 

 material for evolution inasmuch as they are always changes for the 

 worse, due to the loss of factors, and therefore could not furnish ma- 

 terial for progressive evolution; that mutations are unadaptive in 

 character and therefore fail to help in the solution of the problem of 

 adaptation; and that, even if most of the Lamarckian experiments 

 have given negative results during the few months or years during 

 which they have been carried on, this does not mean that no changes 

 would take place in much longer periods of time. 



The present controversy between the Lamarckians and the muta- 

 tionists is practically the same as the perennial conflict between 

 Lamarckians and Darwinians, for the mutationists rely upon natural 

 selection to explain both progressive evolution and adaptation. Muta- 

 tions form the raw material for selection instead of the fluctuating 

 variations of Darwin, which are now believed to be chiefly somatic 

 modifications. The pure-line experiments of Johanssen and others, 

 described in the last chapter, may be said to indicate that selection on 

 the basis of somatic modifications (acquired characters) gives only nega- 

 tive results, while germinal potentiality remains quite unchanged 

 under all sorts of somatic conditions. It appears that, in general, the 

 germ plasm is remarkably stable, and that when changes do occur 

 they occur abruptly. Thus pure-line work seems to negative the effec- 

 tiveness of two alleged agencies in evolution, namely, the Darwinian 

 type of selection based on fluctuations, and the inheritance of somatic 

 modifications. 



The next two rather long chapters present critical discussions of 

 the rival hypotheses (the Lamarckian hypothesis and the mutation 

 hypothesis), and in this place we shall merely make a brief statement 

 of the historical background of each hypothesis and its present status. 



The Lamarckian hypothesis, past and present. — A review of 

 chapter ii will serve to show that the idea that the effects of use and 

 disuse are inherited dates back at least as far as Aristotle, that it was 

 more or less taken for granted by all evolutionists up to the time of 

 Lamarck; that Lamarck put in definite terms what had been the 

 prevailing belief of thinking people; that Erasmus Darwin (before 

 Lamarck) and Charles Darwin (after Lamarck) believed in the in- 

 heritance of the effects of use and disuse, and that it was not until 

 Weismann attacked the theory that there was any noteworthy opposi- 

 tion to it. Weismann's germ-plasm theory, especially his ideas of the 



