CHAPTER V 

 VARIABILITY 



Variations as distinguished from modifications Spontaneous variations as 

 distinguished from those due to the direct action of the environment Evidence 

 of the existence of the latter Discussion of the evidence Variability in uni- 

 cellular and multicellular organisms. The insusceptibility of the germ-plasm 

 to direct action of the environment The fallacies of experimental evidence 

 Variation in new environments Nearly all variations are spontaneous The cause 

 of spontaneous variations The three fundamental laws of heredity. 



w : 



125. "^H 7"E dismiss, then, the Lamarckian doctrine. The only 

 intelligible theory of evolution remaining is that 

 of Natural Selection. Our task, then, is to ascer- 

 tain^ on the one hand, whether this doctrine accords with all the 

 facts of adaptation is contradicted by none and, on the other 

 hand, to discover the theory of heredity which accords with it. 

 Since we are able to account for innate likenesses between parents 

 and offspring, 1 we have, in effect, to study the origin of innate 

 differences, of variations. 



126. The reader may be reminded that a variation in the 

 sense in which we use the word, which is that of common scientific 

 usage is a difference between parents and offspring, due to a differ- 

 ence in germ-plasms. It is not a 'modification,' a difference 

 caused by an unequal play of stimuli on parent and child. While 

 biologists still believed in the transmission of acquirements, the 

 term 'variation' was used to indicate any difference, whether 

 germinal or due to an unequal play of stimuli. But, when the 

 Lamarckian theory was being discarded, and the importance of 

 distinguishing between the two kinds of differences was realized, 

 a tendency to limit the term to differences founded on changes in 

 the germ-plasm became general. Even now, however, the word is 

 sometimes given unlike meanings by various writers. Thus some 

 authors, still using it in its old sense, distinguish by means of an 

 adjective between " blastogenetic, genetic, or germinal variations " 

 (i.e. variations properly so called) and "somatogenic variations" 

 (i.e. modifications). Others imply that any change in the child 



1 See 88. 

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