54 EVOLUTION AND ANIMAL LIFE 



we are for the moment interested in. The offspring may show 

 a blend of the different characters of the parents, or a mosaic 

 of them, or may show the characters of either one alone, or, 

 indeed, characters of wholly new type. The important thing 

 is, however, that there is no certainty indeed there is almost 

 certainty of the opposite that any particular variation will be 

 fostered and fixed if miscellaneous interbreeding is allowed. 

 So that a segregation of individuals having certain common 

 variations or varietal characters is necessary for the perpetua- 

 tion of these characters. 



Now the most usual way, probably, in which this segregation 

 or isolation is brought about is by topographic or geographic 

 barriers ; a group of individuals gets isolated from others of their 

 species by some physical barrier, and the variations that appear 

 among them, due often to some cause incident to the special 

 locality and hence common to all of them, are readily preserved 

 and fostered by the enforced breeding among themselves. 

 But such an isolation may conceivably be brought about in 

 several other ways, and observation has shown that probably in 

 some cases so-called biologic isolation occurs, that is, that a 

 restriction of miscellaneous interbreeding among individuals of 

 one species, and an enforced selective breeding among certain 

 ones possessing certain variations or differences in common, 

 does really obtain. Such isolation is also called physiologic, or 

 sexual, isolation. 



Many biologists, and the number of them has increased 

 rapidly in the last few years, due primarily to the activity and 

 leadership of the botanist de Vries (Amsterdam), believe that 

 species-forming is achieved without the aid of the selection 

 factor; that the actual production of species is a function of 

 variation ("mutation' the special kind of variation efficient 

 in species-making is called), and that the influence of selection 

 is only of a more remote and generally restraining, and thus 

 directive, nature. Such biologists may be said to believe in 

 species-forming by heterogenesis or saltation, as contrasted 

 with species-making by slow, gradual transmutation. And 

 de Vries and his followers have adduced a few apparently 

 undeniable examples of species-forming by heterogenesis. At 

 least this influence seems to have produced forms to all in- 

 tents and purposes apparently similar to natural species. So 

 the particular kind of variation called mutation, which is the 



