MORE ABOUT SPECIES 105 



impossible to make observations with regard to "pairing" 

 and "breeding true." 



Some 400 species of fleas have been described, and 

 we are certain as to the value of the characters relied on 

 to distinguish those species, owing to what we know of 

 the breeding of some common species of fleas. The flea 

 of the domestic fowl, that of the domestic pigeon, that 

 from the house-martin, and that from the sand-martin — 

 used to be considered as one species until they were 

 carefully examined twenty years ago. In reality each of 

 them has its own peculiar " marks," and they do not mix 

 with one another. The nests of the sand-martin yield 

 only one species of flea, namely that peculiar to the 

 sand-martin. The hen-house, the dove-cote, and the 

 nests of the house-martin yield each their flea maggots, 

 which can be reared and become in each case a 

 distinct species with definite recognizable " characters." 

 On the other hand, the flea of the rabbit gives an 

 opportunity of studying the limits of variation in a 

 " good " species. Rabbit warrens swarm with the rabbit 

 flea, and often a great number are found on one 

 rabbit, the individual fleas "varying" — "differing" from 

 one another to a slight extent. The "systematist " thus 

 gets to know what organs are variable within the limits 

 of an undoubted physiological species of flea, and what 

 are comparatively constant — so that he can form a 

 reasonable opinion about the claim of other specimens 

 which he may receive without full history of their habits, 

 to be regarded as true distinct species. 



The fact that most important chemical differences of the 

 blood and digestive juices often accompany the small ex- 

 ternal differences which enable us to distinguish one species 

 of animal or plant from another, makes it obvious that 

 the knowledge of species is a very valuable and necessary 

 thing. One species of flea, the Pulex Cheopis, habitually 



