18 MANUAL CF ZOOLOGY. 



garded merely as a convenient, but not immutable, abstrac- 

 tion, the employment of which is necessitated by the require- 

 ments of classification. 



By Buffon, ' species ' is denned as ' a constant succession 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 de- 

 scended, or may be regarded as being descended, from a single 

 primitive pair by an uninterrupted succession of families.' 



Miiller defines species as ' a living form, represented by in- 

 dividual beings, which reappears in the product of genera- 

 tion with certain invariable characters, and is constantly 

 reproduced by the generative act of similar individuals,' 



According to Woodward, 'all the specimens, or individuals, 

 which are so much alike that we may reasonably believe them 

 to have descended from a common stock, constitute a species. 1 



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 exactly simi- 

 lar to one another. The characters in which individuals 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, and relations to 

 surrounding circumstances and external objects.' 



On a closer examination, however, it will be found that 

 these two leading ideas in the definition of species external 

 resemblance and community of descent are both defective, 

 and liable to break down if rigidly applied. Thus, there are 

 in nature no assemblages of plants or animals, usually 

 grouped together into a single species, the individuals of which 

 exactly resemble one another in every point. Every na- 

 turalist is compelled to admit that the individuals which com- 

 pose any so-called species, whether of plants or animals, differ 

 from one another to a greater or less extent, and in respects 

 which may be regarded as more or less important. The exist- 



* In using the term ' individual,' it must be borne in mind that the 'zoo- 

 logical individual' is meant; that is to say, the total result, of the develop- 

 ment of a single ovum, as will be hereafter explained at greater length. 



