SPECIES AND THEIR ORIGIN 323 



figurative sense. It is the essential nature of H 2 O 

 to be a solid at low temperature. In the same way 

 the climatic conditions of the mountains do not 

 cause the profound .modifications in the habit of 

 alpine plants in contrast to their congeners of the 

 lowlands. The nature of the species is to respond, 

 morphogenetically, in one way to one sort of environ- 

 ment, and in another way to another sort of environ- 

 iiK'iit without being intrinsically altered by either. 

 For the succession of individuals is a continuous 

 stream, and there is no absolute break between one 

 generation and another. In other words the species, 

 like the organism, has a unity unaffected by its sur- 

 roundings. This consideration does not affect the 

 idea that species do alter with respect to their in- 

 trinsic nature, and that in the course of time new 

 species come into existence from preexisting ones. 

 It emphasizes, however, the significance of the 

 internal factors involved and the relative unimpor- 

 tance (in a direct and permanent way) of the action 

 of the environment. 



In conclusion it may be said that biologists are by 

 no means so positive in giving their allegiance to one 

 theory or another of the origin of species as they 

 might have been a generation ago. The concept 

 of Evolution, that is, of the progressive changeful- 

 ness of organic Nature and the descent of present- 

 day species by modification of preexisting types, 

 forms the basis of all modern biological work. 

 As to the method of the evolutionary process there 

 are several opinions, and it may very well be that 



