278 Practical Plant Biology. 



variations are adequate to account for the transformations of 

 species. 



The older attempt, associated with the name of Lamarck, to 

 explain the transformation of species, assumed that, as in the indi- 

 vidual, the use of a part secures its perfection and development, 

 while if it falls into disuse, atrophy and reduction overtakes it, 

 so in the race those parts which were used and exercised are de- 

 veloped while those which are superfluous and disused disappear 

 and atrophy. The characters produced by exercise are evidently 

 characters acquired by the individual, and in proportion as experi- 

 mental evidence has accumulated showing that such characters are 

 rarely if ever transmitted, Lamarck's theory of the transformation of 

 species has lost ground. 



Another possible cause contributing to the transformation of 

 species is the segregation of determinants in the formation of the 

 germ cells, and the new combinations of characters resulting from 

 fertilisation revealed by research following Mendel's discoveries. 

 But here again, although certainly new races and varieties are 

 formed by this process, all naturalists are not convinced that the 

 characters produced, or made apparent, in this way are of sufficient 

 importance to transgress the limits of species and genera. 



Thus while the theory of evolution, or the origin of species 

 by descent, stands established by the consistent evidence of many 

 lines of research, the method by which evolution acts remains a 

 mystery. Towards the solution of this mystery we possess guiding 

 suggestions, it is true ; but whether they will prove adequate, or 

 will have to give place to others when knowledge is extended, time 

 alone can show. 



