WHAT IS A SPECIES? 817 



Many of the species included within the larger genera 

 resemble varieties in being very closely, but nnequally, 

 related to each other, and in having restricted ranges 

 (Darwin). 



Bateson divides variation into continuous or gradualj 

 and discontinuous or sudden, modifications, as sports, de- 

 formities, etc. Mcristic variation deals with the change 

 in the number of successive j)arts, and substantive variatio7i 

 with the chemical modification of parts. Variation is also 

 said to be either fortuitous or aimless, or determinate and 

 adaptive, leading to a definite end. For example, in the 

 Cyprinoid fishes of tlie streams of the Pacific slope variation 

 extends along definite and parallel lines; these are directed 

 towards an increase of fin-rays and towards a modification 

 of rays into sj^ines (Eigenmann). 



What is a Species? — As every one knows, no two indi- 

 viduals are alike, neither parents nor offspring. Every 

 naturalist has found by experience that in many forms it is 

 impossible to draw the line or to establish the limits between 

 species and varieties. It is owing to the variation of 

 species of plants and animals that naturalists liavo been 

 led to believe that species are arbitrary or ideal collections 

 of individuals, and to conclude that the individual oidy is 

 a fact, and that the species is an induction, an ideal group 

 of individuals. 



Of the many definitions of species that of Lamarck 

 is the most satisfactory: "A species is a collection of 

 similar individuals which are perpetuated by generation in 

 the same condition as long as their enviro7iment has not 

 changed sufficiently to bring about variation in their 

 habits, their character, and their forms." 



Darwin's idea of species is as follows: "I look at the 

 term species as one arbitrarily given for the sake of con- 

 venience 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 fluc- 

 tuating forms. The term variety, again, in comparison 



