HISTORY OF ZOOLOGY. 25 



porters. Darwinism was long in obtaining an entrance into 

 France. 



DARWIN'S THEORY OF THE ORIGIN OF SPECIES. 



Before Darwin wrote the idea of fixity of species prevailed 

 among systematists. It was recognized that all the individuals of 

 a species are not alike, and that more or less pronounced variability 

 occurs, so that it was possible to distinguish races and varieties 

 within the species, but it was believed that the variations never 

 transcended specific bounds. 



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 determining 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 between Species and 

 Variety. For the settlement of this fundamental question morpho- 

 logical 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 be obliterated by no inter- 

 mediate 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 when they lose their 

 importance through the existence of intermediate forms. A 

 definite application of this rule discloses great incongruities, many 

 animal and vegetable groups being regarded by one set of systema- 

 tists as good species, by another only as 'sports/ i.e., as varieties 

 of the same species. The differences between the ' sports ' of our 

 domestic animals are in many instances so considerable that 

 formerly they were regarded not only as sufficient for the founda- 

 tion of good species, but even of genera and families. In the 

 fantail pigeon the number of tail-feathers, formerly only 12-14, 

 has increased to 30-42 (fig. Ic) ; among the other races of pigeons 



