WHAT IS A SPECIES? 155 



only the very simplest and lowest animals came into exist- 

 ence ; those of a more complex organization only at a later 

 period. 



The Fundamental Tenet of the Mutability School. Thus we 

 find that the fundamental difference between the hypothesis 

 of the "immutability of species" of Cuvier, and that of the 

 " mutability of species" of Geoffrey St. Hilaire, Lamarck, 

 and Darwin, consists in the assumption by the more modern 

 school that the specific morphological characters of organisms 

 are temporary ; are constantly undergoing slight modification 

 from generation to generation; and, finally, that separate 

 species are not such from the beginning, but take their place in 

 an orderly sequence of phenomena ; that which constitutes the 

 specific character for each case having an explanation in what 

 preceded it, and bearing the relation of cause, or taking a part in 

 determining what shall follow. 



The removal of "special creation" from the one theory 

 and " spontaneous generation " from the other was the natural 

 result of the progress of ideas, an opening of the laws of 

 organic evolution to scientific investigation. These two 

 hypotheses were the natural recourses of ignorance, and the 

 present form of our philosophy is no less obliged to find an 

 unobservable origin for the things of whose existence we 

 have observable evidence. 



State of Opinions when Darwin began his Investigation of the 

 Origin of Species. This brings us to the stage in the history 

 of opinions when Darwin began his investigations. The 

 mutability of species had been announced and strongly sup- 

 ported by able advocates. The general principle of evolu- 

 tion had been formulated centuries before, but was rather in 

 the stage of speculative opinion than applied hypothesis ; the 

 facts supporting and illustrating it were not greatly accumu- 

 lated. Linn, Cuvier, and their schools had already defined a 

 great number of species of plants and animals, had classified 

 them, and had erected an elaborate systematic botany and a 

 systematic zoology on the theory of immutability of species. 

 The new theory seemed to shake the foundation of the science 

 of Natural History. If there is no fixity to the idea of 

 species, the query arose, what can we talk about ? What is 



