Nature of Heritable Variations 83 



clear material spread through the cytoplasm; this is known 

 as a chromidium. Unfortunately these things are not yet 

 thoroughly known for Difflugia itself, but in some apparently 

 close relatives, such as Arcella (Figure 26), they have 

 been much studied. In the division of these organisms the 

 nuclei divide with the same minute precision that is evident 

 in higher creatures. But the chromidial masses merely sep- 

 arate loosely into halves, with no indication of precision. So 

 possibly the offspring may get different parts of the 

 chromidium in different cases, and it has been suggested that 



Figure 26. Arcella vulgaris, to show the two nuclei (N), and the 

 chromidium (C), or loose cloud of nuclear material. After Hertwig. 



the differences that arise in the hereditary characters are 

 due to this inexactness of division. But this is all specula- 

 tion as yet, without much foundation of probability. 



But in any case it appears to me that these details do not 

 affect the main fact, which is that in these organisms gradual 

 inherited variations are occurring, so that in the course of 

 time many hereditarily diverse families arise from one. In 

 other words, if we study these organisms with sufficient mi- 

 nuteness and perseverance, we see evolution occurring. 



We are now in position to sum up the facts as to heredity 

 and variation in these animals when they are reproducing 

 from a single parent. Any species consists of a great num- 



