ARTIFICIAL AND NATURAL SELECTION 33 



be allowed to apply them here to that process, too, which corre- 

 sponds to the variety-testing of the breeders. 



As the easiest and most notorious examples we choose the whitlow- 

 grasses, or Draba verna, and the wild pansies, or Viola tricolor. Na- 

 ture has constituted them as groups of highly different constant 

 forms, quite in the same way as wheat and corn. Assuming that 

 this has happened long ago somewhere in central Europe, it is, of 

 course, probable that the same differences in respect to the influence 

 of climatic conditions will have prevailed as with grains. Subsequent 

 to the period which has produced the numerous elementary species 

 of the whitlow-grass, came a period of geographical dispersion. This 

 process must have been wholly comparable with that of acclimatiza- 

 tion. Some forms must have been more suited to northern climates, 

 others to the soils of western or eastern regions, and so on. These 

 qualities must have decided the broad lines of the dispersion, and 

 the species must have been segregated according to their respect- 

 ive climatic peculiarities and their claims on soil and weather. A 

 struggle for life and a natural selection must have accompanied and 

 guided the dispersion, but there is no reason to assume that the sun- 

 dry forms should have been changed by this process, and that we 

 see them now endowed with other qualities than were theirs at the 

 outset. 



If this sketch strikes you favorably, natural selection must have 

 played the same part in a large number of other cases, too. Indeed, 

 it may be surmised that this has been its chief and prominent func- 

 tion. Taking up again our image of the sieve, we may assert that in 

 such cases climate and soil are the sifting agents, and in this way 

 the meaning of the image at once becomes a more definite one. Of 

 course, next to climate and soil come the biological conditions, the 

 vegetable and animal enemies of the plants, and other influences of 

 the same nature. 



Thus, everywhere in nature there must be a struggle for life in 

 which closely related elementary species are competing with one 

 another, fighting the same enemies. Some succeed, whilst others fail. 

 Nature in this way performs her primary selection, and hence this 

 process can be called selection between elementary species, or inter- 

 specific selection. 



The alternate principle could then be called the selection wdthin 

 the elementary species, or the intraspecific selection. It has now 

 more closely to be considered. First of all comes the question 

 whether it plays a prominent or only a subordinate part in nature. 



This question may be reduced to another form, in which it is more 

 accessible to direct investigation. Species, as we see them in nature, 

 are in the main constant forms, fluctuating within distinct limits, 

 which are not seen to be transgressed. Now the question arises, 



