CLASSIFICATION 365 



kind, and the average observer familiar only with the better 

 known fishes will perhaps say that he experiences little difficulty 

 in arranging these into more or less well-defined groups. He 

 will be able to distinguish, say, a Roach firom a Bream, Chub, or 

 Barbel, and something in the general appearance of these fishes 

 will suggest that they are fairly closely related to one another, 

 but widely separated from such forms as the Salmon or Perch. 

 The task of the systematist, studying every kind of fish from all 

 parts of the world, is by no means as easy as this, however, 

 and he may experience considerable difficulty in distinguishing 

 the species in some groups of fishes, besides constantly commg 

 across intermediate forms or connecting links between two 



species. 



A great deal has been written, not only on the subject ot what 

 does and what does not constitute a species, but also on the 

 very vexed problem of the manner in which new species first 

 come into being. It is beyond the scope of this work to enter 

 into such controversial matters in any detail, but since the 

 term species has been used fairly frequently, some sort of 

 explanation of its meaning seems necessary. Of the various 

 definitions which have been advanced, that given by Dr. Tate 

 Regan to the British Association during their meeting at 

 Southampton in 1925 seems to come nearest to the truth, and 

 is one which would probably meet with the approval of a large 

 number of systematic zoologists. "A species," he says, "is a 

 community or a number of related communities, whose dis- 

 tinctive morphological characters are, in the opinion of a 

 competent systematist, sufficiently definite to entitle it, or them, 

 to a specific name." By the term community is meant a 

 collection of individuals such as occurs in nature, with similar 

 habits, which live together in a certain area and breed freely 

 with one another. It is assumed that all the individuals 

 composing a species are mutually fertile and produce offspring 

 more or less like themselves, and if this can be proved to be not 

 the case it may generally be taken for granted that two or 

 more species have been confounded with one another. It has 

 been held by some authorities that steriUty, in the sense ot 

 incapacity to produce fertile oflfspring when crossed with 

 another form, is a definite test of true species as opposed to mere 

 varieties, but it not infrequently happens that crosses between 

 two forms which no systematist would hesitate to regard as 

 other than distinct species produce fertile hybrids [cf. p. 339). 

 Other attempts to draw up stricter definitions of a species have 



