VARIATION AND HEREDITY 209 



reddish veins in the leaves, oblonga by narrow leaves, 

 and albida by whitish ones. Gigas had stems nearly 

 twice as thick as lamarckiana, covered with dense 

 foliage. All of these forms reappeared in subsequent 

 cultures (compare the accompanying table), and all 

 bred true to type. 



De Vries also found two plants of lata growing 

 wild in the field, as well as one of a different type 

 with smooth leaves, which he called loevifolia. 

 This form not only bred true when segregated, but 

 developed other types, lata, nanella, rubrinervis, and 

 two others that had not appeared from the lamarck- 

 iana seed. 



The various types of evening primroses described 

 above are good examples of discontinuous variations 

 which have been watched, as it were, in the making. 

 Such variations De Vries believes to be very different 

 in nature from ordinary fluctuating variations, and 

 to them he gave the name mutation. 



Correlated Variation. Since all the parts of a 

 normal organism, at least of an animal, function as a 

 unit, this harmony of action demands a somewhat 

 similar harmony of structure. We should expect 

 to find that organs which function together would 

 vary together. The analysis of comparative measure- 

 ments proves this to be a fact. The abstract index 

 of variation of one organ may be compared with 

 that of another in order to get a single coefficient 

 oj correlation (usually designated " r "). The 

 methods for securing this result are somewhat com- 



