3O GENERAL PRINCIPLES OF ZOOLOGY. 



other energetic defenders of the theory in Germany should 

 be mentioned Fritz Miiller, Carl Vogt, Weismann, Moritz 

 Wagner, and Nageli, even if they have assumed special 

 standpoints in reference to the causes conditioning the 

 changes of form. Among the English investigators of 

 nature are to be named particularly Huxley, Hooker, and 

 Lyell. Darwinism obtained latest an entrance into France. 



DARWIN'S THEORY OF THE ORIGIN OF SPECIES. 



The Problem Stated. Darwin begins with a criticism 

 of the term species. Is the conception of species on the 

 one side and that of race and variety on the other something 

 entirely different? Are there special criteria for deter- 

 mining beyond the possibility of a doubt whether in a 

 definite case we have to do with a variety of a species or 

 with a different species ? or do the conceptions in nature 

 pass into one another? Are species varieties which have 

 become constant, and precisely in the same manner are 

 varieties species in the process of formation? 



Morphological Characters. A. Distinction bet'cveen 

 Species and ]^aricty. -For the settlement of this fundamental 

 question morphological and physiological characters can be 

 considered. In the practice of the systematists usually the 

 morphological characters prevail exclusively ; for that reason 

 they will be here considered first. If, among a great number 

 of forms similar to one another, two groups can be adduced 

 which differ considerably from one another, if the difference 

 between them is obliterated by no intermediate forms, and 

 if in several successive generations they remain constant, 

 then the systematist speaks of a good species ; on the other 

 hand he speaks of varieties of the same species when the 

 differences are slight and inconstant, and still further 

 when they lose their importance through the existence of 

 intermediate forms. A definite application of this rule 

 discloses great incongruities, wherewith is connected the 

 fact that many animal and vegetable groups are by one set 

 of systematists regarded as good species, by another set 



