DEFINITION OF SPECIES 



223 



to represent the limits of an existing species. Within each one 

 the individuals differ but slightly from one another, not more, 

 perhaps, than might be expected amongst the offspring of the 

 same parents. The three species represented have, however, 

 become more or less widely separated from one another by the 

 dying away of the branches from which they arose, and which 

 are represented in the diagram by dotted lines. 



Darwin has told us that he looked " at the term species as 

 one arbitrarily given, for the sake of convenience, to a set of 

 individuals closely resembling each other, and that it does not 

 essentially differ from the 

 term variety, which is 

 given to less distinct and 

 more fluctuating forms. 

 The term variety, again, 

 in comparison with mere 

 individual differences, is 

 also applied arbitrarily, for 

 convenience' sake." l 



This, of course, is no defi- 

 nition of the term species 

 and was not intended as 

 such. A definition in ac- 

 cordance with the above 

 views might, however, be 

 given as follows : " A 



species is a group of FiG. 85. Diagram to illustrate the Separa- 

 individuals that closely 

 resemble one another 

 owing to their descent from common ancestors, which has 

 become more or less sharply separated from all other co-existing 

 species by the disappearance of intermediate forms." Such a 

 definition would be in accord with the practice of many systematic 

 naturalists, who are in the habit of uniting species previously 

 considered as distinct whenever intermediate forms are found, 

 the former species being reduced to the rank of varieties of one 

 and the same species. 



It is obvious that the element of time must be taken into 

 consideration in forming such a conception of species. At any 



1 ( )i ijin of Species." Ed. 6, p. 42. The reader should compare the very similar 

 views ( ,f Lamarck on the species question, given in Chapter XXIV. 



tion of Species by the Dying out of 

 connecting Links. 



