1 62 Changing Plant and Animal Nature 



binations of structural variations of the brain and nervous 

 system, of the sense organs, and of chemical or functional 

 differences of the various tissues, including the so-called 

 ductless glands. 



We may say then that in general there are individual 

 variations within a given group of plants or animals both in 

 respect to the chemical and functional characteristics and 

 in respect to the physical and mechanical characteristics. In 

 any given set of observations, we are likely to find a con- 

 tinuity of characteristics from one extreme to the other with 

 a tendency for more and more individuals to cluster around 

 the mode or typical measurement. Variations of this type 

 have accordingly been designated as continuous variations, 

 or fluctuations. With regard to our idea of species, the as- 

 sumption has been made that individual variations are quite 

 normal so long as they confine themselves to these limits. 

 The concept of fixity of species assumes that while there is a 

 great deal of fluctuation in all directions (that is to say, with 

 respect to every character) , the individual of a given species 

 never transcends these limits. The problem of evolution is 

 to consider (i) whether as a matter of fact any individuals 

 of known ancestry do or do not ever transcend these sup- 

 posed limits of fluctuation, and (2) whether there is any 

 intergradation between one supposed species and another. 



Sources of Yariation 



We do not know in general what causes one individual 

 to differ from another of the same species or of the same 

 parentage. Many possibilities suggest themselves. We may 

 think of the plastic organism being forced in the course of 

 its development to assume one character or another by the 

 external conditions. The distortions of the human figure 

 imposed by various occupations might be an example of 

 such a process. We might suppose that the response of the 

 organism to varying stimulations would bring about di- 

 verging end results. Children exposed to sunlight become 



