290 Evolution and Adaptation 



different elementary species of Draba verna are less different 

 from each other than the forms of leaves on a tree. The_ 

 essential differences between the two kinds of variation is 

 that the mutation is constant, while the continuous variation 

 fluctuates back and forth. "• 



The following example is given by De Vries to illustrate the 

 general point of view in regard to varieties and species. The 

 species Oxalis corniculata is a " collective " species that lives 

 in New Zealand. It has been described as having seven well- 

 characterized varieties which do not live together or have in- 

 termediate forms. If we knew only this group, there would 

 be no question that there are seven good species. But in 

 other countries intermediate forms exist, which exactly bridge 

 over the differences between the seven New Zealand forms. 

 For this reason all the forms have been united in a single 

 species. 



Another example is that of the fern, Lomaria procera, from 

 New Zealand, Australia, South Africa, and South America. 

 If the forms from only one country be considered, they appear 

 to be different species ; but if all the forms from the different 

 parts of the world be taken into account, they constitute a 

 connected group, and are united into one large species. 



It will be seen, therefore, that the limits of a collective 

 species are determined solely by the deficiencies in the ge- 

 nealogical tree of the elementary species. If all the element- 

 ary species in one country were destroyed, then the forms 

 living in other countries that had been previously held 

 together because of those which have now been destroyed, 

 would, after the destruction, become true species. In other 

 words : " The Linnasan species are formed by the disap- 

 pearance of other elementary species, which at first connected 

 all forms. This mode of origin is a purely historical process, 

 and can never become the subject of experimental investi- 

 gation." Spencer's famous expression, the " survival of the 

 fittest," is incomplete, and should read the " survival of the 



