VARIATION 415 



which result in somatic mutations? "As regards the ulti- 

 mate nature of mutations, we are inclined to look upon them 

 as the result of various types of change in the nucleus: (1) 

 morphological changes (a) in number, (6) in shape and 

 size of the chromosomes, or in the arrangement of their 

 substance; (2) chemical or functional changes in (a) whole 

 chromosomes or (h) portions of particular chromosomes, by 

 which a function may be modified or lost; (3) two simulta- 

 neous mutations may occur through mismating of the chromo- 

 somes in two pairs so that each germ-cell receives both 

 members of one pair; (4) changes in the mysterious karyo- 

 lymph or gel which forms the groundwork of the nucleus. 

 Such changes may be thought of as alterations in chemical 

 structure or even in polarity, and may also be supposed to 

 extend to the ground substance of the whole cell. But the 

 real nature of all such changes as those last mentioned is 

 at present highly speculative" (1915, p. 303). 



§ 4. Prohlem of the Origin of Variations. 



Turning now to the problem of the origin of inborn varia- 

 tions, we may usefully distinguish two levels of difficulty. 

 There are variations and variations. There are some novel- 

 ties that imply just a little more or a little less of some 

 quality,— a slightly longer tail, a slightly denser blackness, 

 a slightly stronger flight-muscle, a slightly weaker eye; some 

 that involve a disappearance of an entire character, such 

 as hair or horns, tail or pigment; some that may be described 

 as obvious re-arrangements of the characters displayed by 

 the ancestry, as we see in a piebald pony or in a hybrid cock- 

 atoo. Now it does not seem very difficult to imagine the 

 origin of this kind of quantitative variation. Without pin- 

 ning our faith as yet to any very detailed view of the ma- 



