58 ELEMENTS OF BIOLOGY. ] 



of individuals"^ similar to and capable of reproducing each 

 other." 



De Candolle defines species as an assemblage of all those 

 individuals which resemble each other more than they do 

 others, and are able to reproduce their like, doing so by the 

 generative process, and in such a manner that they may be 

 supposed by analogy to have all descended from a single 

 being or a single pair. 



M. de Quatrefages defines species as "an assemblage 

 of individuals, more or less resembling one another, which 

 are descended, or m^ay be regarded as being descended, 

 from a single primitive pair by an uninterrupted succession 

 of families." 



Muller defines species as "a living form, represented by 

 individual beings, which reappears in the product of gener- 

 ation with certain invariable characters, and is constantly 

 reproduced by the generative act of similar individuals." 



According to Woodward, "all the specimens, or indi- 

 viduals, which are so much alike that we may reasonably 

 believe them to have descended from a common stock, con- 

 stitute a speciesJ^ 



From the above definitions it will be at once evident that 

 there are two leading ideas in the minds of zoologists when 

 they employ the term species ; one of these being a certain 

 amount of resemblance between individuals, and the other 

 being the proof that the individuals so resembling each 

 other have descended from a single pair, or from pairs ex- 

 actly similar to one another. The characters in which indi- 

 viduals must resemble one another in order to entitle them 

 to be grouped in a separate species, according to Agassiz, 

 " are only those determining size, proportion, colour, habits. 



* In usinG: the term "individual," it must be borne in mind that the 

 "zoological individual" is meant; that is to say, the total result of the 

 development of a single ovum^ as will be hereafter explained at greater 

 length. 



