!$ VARIATION 



is it a difference in kind (qualitative) ? For example, one horse 

 is exactly like another, only larger : the difference is quanti- 

 tative. Another is no larger, but he can draw more and has 

 greater endurance : the difference is qualitative. One cow gives 

 more milk than another : the difference is quantitative ; but a 

 third gives better milk, and the difference is qualitative. One 

 apple is larger than another of the same variety (quantitative 

 variation), but another is different in texture and flavor (quali- 

 tative variation). 



When, therefore, in the study of a racial character as repre- 

 sented in the same or in different individuals it is found to have 

 varied, the first question to ask and answer is this : Is the devi- 

 ation one in kind or merely in amount ? Is it qualitative or merely 

 quantitative ? Is the change to be regarded as one in nature or 

 only in degree ? If the student will carry these distinctions always 

 in mind, he will avoid much needless confusion. 



Variation continuous and discontinuous. It is not to be assumed 

 that variations differ from one another by infinitesimal increments. 

 The differences may be infinitesimal (continuous variation) or they 

 may be "discrete " (discontinuous variation). 



Darwin supposed, and it is commonly assumed, that variation 

 is by nature continuous, and that new forms originate by the 

 gradual accumulation of insensible differences through the agency 

 of long-continued selection. This means that if all the individuals 

 that ever lived could be assembled and so assorted as to bring 

 nearest together those that are nearest alike, it would then be 

 found that they would grade into one another by imperceptible 

 differences, and that any gaps that might occur would be due to 

 the effect of selection in blotting out intermediate forms. 



Now this is a hasty assumption, indicating in these days but 

 a superficial acquaintance with the manner of variation. We 

 cannot assume that all possible values in variable characters are 

 presented ; indeed, we know very well that in many cases all 

 possible values are not presented, and that some intermediate 

 forms never arise. For example, peaches often give rise to 

 nectarines, but there is a gap between the two that apparently is 

 never filled. Darwin called an occurrence of this kind a "sport," 

 as if it were an instance in which all ordinary laws were set aside, 



