THE HISTOEY OF SPECIES. 



1. THE NATURE OF SPECIES. 



Definition of Species — Specific Constitution of Protoplasm. 



DEFINITION OF SPECIES. 



The history of plant species is founded on the history of individual plants 

 given in the first section of this volume, more particularly on the results afforded 

 by investigation into the processes of reproduction and propagation. It deals with 

 the description of the species from its origin to its end, and also takes cognizance of 

 the replacing of extinct species by new ones. The execution of this task is less easy 

 than the representation of the life-history of the individual which can be deduced 

 from direct observation and experience. It being possible to follow the course of 

 even long-lived individuals, beginning with the origin of the embryo and following 

 it through all its life's stages, the meaning of certain vital processes as, for example, 

 the pollination of the stigmas and the germination of the seeds can be understood. 

 The origin of most of our present species, however, which have arisen without the 

 aid of man, is shrouded in mystery; it occurred in long past ages and we are 

 obliged to fall back on conjectures which, however intelligent and however they 

 may be supported by carefully weighed considerations, are still only conjectures 

 after all. For purposes of direct observation we have only the fossil remains of 

 earlier times and the species which are living at the present day. By comparing 

 these with one another, and by inferring the nature of extinct forms from that of 

 living species, we are able to construct a chain of conclusions which after all may be 

 regarded as the best available substitute for a history of species. 



The most important foundations for these conclusions are afforded by the 

 knowledge of the relations of living species to their environment, especially the 

 recognition of those causes which bring about permanent changes of form, for this 

 alone can elucidate the question of the origin of new species. Before discussing 

 these important questions the nature of species must be described and we must 

 understand exactly what is meant by a species. 



The definition of a species was first introduced into science by Linnseus, and 

 the Latin word " species " owes its origin in this sense to the great master of 

 Botany. Linnaeus laid down that each species consists of similar individuals which 

 are related together by their origin, and which are the unaltered descendants of 

 a common ancestor or pair of ancestors. It does not affect the value of the 



