244 THE CANADIAN NATURALIST. [^^^g- 



guished by certain characters. Yet simple though this at first 

 sight appears, we shall find that many intricate questions are con- 

 nected with it. Our idea of the species is based on the resem- 

 blance of the individuals composing it in all the characters which 

 we consider essential. If, for instance, a number of sheep and 

 goats are placed before us, we readily select the individuals of 

 each species. In doing this we give no regard to differences of 

 sex or age, but put the young and old, the male and female, of 

 each species together. Nor do we pay attention to merely acci- 

 dental difierences : a mutilated or deformed specimen is not on 

 that account separated from its species. Nor do we attach value 

 to characters which experience has proved to vary according to 

 circumstances, and in the same line of descent. Such, for ex- 

 ample, are diiferences of colour, or fineness of the hair or wool. 

 The remaining resemblances and differences are those on which 

 we rely for our determination of the species, and which we term 

 essential. "We shall find that these essential characters of the 

 species are points of structure, proportion of parts, ornamentation, 

 and habits. 



These characters constitute our idea of the species, which we 

 can readily separate from the Individuals composing it. The 

 individuals are temporary, but the species is permanent, being 

 continued through the succession of individuals. If all the adult 

 individuals are alike and indistinguishable from each other, then 

 any one may serve as a specimen of the species. If there are 

 difl"erences of sex or Varieties subordinate to the species, then a 

 suite of specimens showing these will represent the species. The 

 species is thus an assemblage of powers and properties manifested 

 in certain portions of matter called individuals, and which are its 

 temporary representatives. It follows that the species is the true 

 unit of our classification, and that the indefinite multiplication of 

 individuals leaves this unchanged. 



Our idea of the species will however be imperfect if we do not 

 distinctly place before our minds its continued existence in time. 

 This depends on the power of reproduction, whereby the indi- 

 viduals now existing have descended from similar progenitors, and 

 will give birth to successors like themselves. A moment's 

 thought will suffice to show that, independently of this, species 

 could have no real existence in nature. If animals were not 

 reproductive, the species would become extinct after the lapse of a 

 generation. If their reproduction followed no certain law, and 



