THE CAUSES OF VARIATION, 



IN the last few years an ever-increasing amount of atten- 

 tion has been given to the study of the facts of varia- 

 tion, for it has gradually become more and more evident 

 that this forms one of the most important vantage grounds 

 from which the problems of evolution may be attacked. 

 The importance of this study depends not a little on the 

 consideration that it is based upon the collection and deter- 

 mination of sure and solid facts, which do not depend for 

 their veracity upon the validity of any particular hypothesis 

 or theoretical consideration. Even more interesting- in 

 many ways, than the study of the facts of variation is that 

 of the causes of variation. To what are the differences in 

 form and structure, often by no means slight, which indi- 

 viduals of a species show among themselves, due ? Why is 

 it that offspring do not bear an exact resemblance to the 

 physical form of their parents ? 



Into this question of the causes of variation, Darwin, 

 who was the first to show the vital importance of the study 

 of the facts of variation with reference to the problems of 

 evolution, entered but little. He considered that variation 

 " is generally related to the conditions of life to which each 

 species has been exposed during several successive genera- 

 tions," and that "changed conditions act in two ways, 

 directly on the whole organisation, or on certain parts 

 alone, and indirectly through the reproductive system" (i). 

 Another view which has been frequently upheld, especially 

 by certain American scientists, is that variation is the result 

 of an inherent tendency on the part of the organism to vary. 

 More recently it has been shown that the problem of the 

 causes of variation is intimately bound up with the question 

 of sex, for the characters of offspring are produced by a 

 mingling of those of their parents, and this mingling may 

 not be in fixed and exactly similar proportions. Thus 

 Wallace believes diversity of sex to be of primary import- 

 ance as the cause of variation (3). Weismann also is con- 



