I04 SECRETS OF EARTH AND SEA 



together, and prove themselves thus to be entitled to 

 be regarded as local " varieties " or " races," but not as 

 fully-separated true species. 



Thus one sees how difficult it is to have knowledge 

 of the breeding test, even in regard to large animals. 

 It is obvious that the difficulty of obtaining it in 

 regard to the thousands of kinds of minute creatures 

 is much greater. Yet when they say, " This is a distinct 

 species," naturalists do mean that it is not only marked 

 off from other animals or plants most like to it by a 

 certain shape, colour, or other quality or qualities, but 

 that it breeds apart with its own kind and does not 

 naturally hybridize with those other forms most like 

 to it. 



Although the kind of naturalist called a " systematist " 

 who makes it his business to accurately describe and 

 record and distinguish from one another all the existing 

 species of some one group — say, of antelopes, of mice, 

 of flowering plants, of fishes, or of fleas — has only a 

 knowledge in a few instances of the breeding of the 

 organisms which he describes as " distinct species," he yet 

 does know, in regard to some one or more of his species 

 in most groups, the facts of pairing and reproduction, and 

 what are the limits of variation in the markings and other 

 characteristics of at least one or two species definitely 

 submitted to the " breeding test," that is to say, ascertained 

 to be "true physiological species," kept apart by deep- 

 seated chemical differences in their blood and tissues. 

 Hence it is legitimate for him, by careful balancing and 

 consideration of all the facts, to determine — not absolutely, 

 but by analogy — the value to be assigned (whether as 

 indicating true species or merely varieties capable of 

 pairing with the main stock) to points of difference among 

 the specimens of a dead collection brought from some 

 distant land or from some position in which it would be 



