March, 1880.] 217 



WHAT IS MEANT BY THE TERM "SPECIES"? 

 BY J. W. DOUGLAS. 



/ The late G. H. Lewes wrote, "The thijiff species does not exist; the 

 term expresses an abstraction, like Virtue, or Whiteness ; not a definite 

 concrete reality, which can be separated from other things and always 

 be found the same. Nature produces individuals ; these individuals 

 resemble each other in varying degrees ; according to their resemblances 

 we group them together as classes, orders, genera, or species. There 

 is a reality indicated by each term — that is to say, a real relation ; but 

 there is no objective existence of which we could say, This is variable, 

 This is immutable. . . . When zoologists have maintained that 

 species are variable, they have meant that animal forms are variable ; 

 and these variations, gradually accumulating, result at last in such 

 differences as are called specific. . . . Every new form becomes 

 established only through the long and gradual accumulation of minute 

 differences in divergent directions."* Now, I do not contend, norllo 

 U^now it ever has been contended, that a species has a definite objective 

 existence in the sense indicated, yet I hold with Cuvier that "species" 

 expresses a reality, aud not merely an abstraction — a formulated idea 

 of characters believed to constitute a species — ; that is to say, that 

 when individuals are so related that, whatever the range of variation 

 amoug them may be, they breed together consecutively, they are thus 

 a real species. A. E. AVallace says, " a species may be defined as a 

 group of individuals of animals or plants which breed together freely 

 and produce their like."t Within this limit individuals may, and do, 

 Tary to a vast extent in colour, marking, size, and structure, according 

 to, and as a result of, the conditions in which they live. This has been 

 proved by the breeds of animals under domestication ; and among 

 insects — notably Lepidoptera — it is incontestibly demonstrated tha,t 

 individuals reared with others from the same batch of eggs have au- 

 f ered so much, in colour especially, that in former time, when breeding 

 was little practised, such would, if captured at large, have been deemed 

 to belong to different species. Thus Stephens says, J " I conceive that 

 where an insect difers from its nearest congener by some trifling varia- 

 tion of form, combined with a diversity of sculpture, dissimilar bulk, or 

 prevalent discrepancy of colour, either in the disposition of particular 

 markings or in a universal change, we are perfectly justified in consi- 

 dering such examples as distinct species, unless we are enabled by 

 experience to show incontestibly that such is contrary to nature." lie, 



* " studies in Animal Life." 



t " Nineteenth Century," January, 1880. 



t "Systematic Catalogue of Britiah Insect.s," lutroductiou, p. xvi. 



