46 SALMONID^ OF BRITAIN. 



■work of destruction. Frogs will likewise prey on them, also many fish as bull- 

 heads, sticklebacks, perches, eels, riYer trout, and other forms of Salmonidae. 

 Among their invertebrate enemies* which have been recorded, it does not seem 

 improbable that some, as the fresh- water shrimp, Gammarus pulex, may be useful 

 in consuming surplus food, which might otherwise tend to putrefy and cause 

 disease, as well as by eating any dead fry, although they may likewise destroy 

 some few of the living, especially such as are weakly. 



There are numerous diseases alevins and fry are subjected to when artificially 

 reared, among which the following have been enumerated : — Constitutional 

 weakness occasioning arrest of development in the young, which is sometimes 

 induced by breeding from young parents or inter- crossing species ; from the same 

 causes deformities may arise, although some, as spinal affections, may be due to 

 injuries sustained by the embryo in the earlier stages of its development. This 

 also seems to be one predisposing cause to blue-dropsy of the sac ; as well as those 

 cases in which small black eyes, so indicative of bad development, are perceptible 

 in the unhatched young. Too much light, likewise, seems to be injurious to the 

 brain, acting through the medium of the optic nerves. In some instances eggs 

 which had been attacked with fungus will hatch, but the young are mostly 

 weakly: while it is not infrequent to observe the alevin suffocated when attempting 

 to emerge from the egg, or should it succeed in its endeavours, to subsequently 

 die from the effects of weaknesss. The water itself may be injurious, owing to 

 its being too warm, insufficiently aerated or polluted. Crowding the young is 

 likewise a fertile source of disease, while insufficiency of nutriment may occasion 

 starvation. Still excess often sets up fungus, owing to a portion of the unconsumed 

 food remaining in and polluting the water. Sometimes the fry are choked by 

 trying to swallow pieces of meat which are too large, while fungus may show itself 

 among them. 



Considerable attention has been devoted of late years to the hybridization 

 of animals and plants, and many assertions, which formerly passed current as 

 undoubted facts, have been ascertained to be partially or wholly erroneous. At 

 the present day we cannot admit the theory of Bay, that " any two animals that 

 can procreate together, and whose issue can procreate, are specifically the same :" 

 nor the 'statement of the elder Flourens, that hybrids can only be produced 

 between individuals belonging to the same genus. In short it has been conclu- 

 sively proved that hybrid offspring are not invariably sterile,t their degrees of 



* Mr. Daubeny, of Eedhamptou Eeetory, observed in The Field ^March 22nd, 1884, p. 406), as 

 follows : — " The most deadly enemy to young fish I believe to be leeches (Piscicola geometra), and 

 in some streams it seems to me a puzzle how any of the fry, when just emerged from the egg, can 

 escape them. The ponds in my garden and the streams round are alive with leeches about one- 

 third of an inch in length : they devour everything. It is impossible to set a night-line. A 

 large lob-worm is eaten in a few minutes by them ; and even a mouse used as a bait by one of 

 my boys for eels had nothing but the tail left the next morning. A piece of meat an inch square, 

 after being left in the water an hour, was found to have 1200 attached to it. On a warm day 

 they may be seen crawling on the bottom of the water in every direction. Hardly a square inch 

 this time of year but has a leech or two on it. I think that they will not touch trout ova, the 

 shell being too tough for them, but fry a few days old, on being placed in a saucer, with some of 

 these tiny leeches by way of an experiment, were at once seized upon and devoured. Their powers 

 of locomotion are too feeble to afford them a chance of escape from their ubiquitous foes." 



t Hybridization has been observed between two quadruma pertaining to different families of 

 Catarhina. Thus an offspring was born in the Eegent's Park Zoological Gardens, on October 

 13th, 1878, which was a cross between a male ape Macacus cynologus and a female baboon 

 Cxjnocephalus mormon. It died December 20th, 1879. 



All are aware of the existence of mules and hiunies between the horse and the ass, and 

 although some of these mules have been observed to produce offspring. Columella (M. de la 

 Malle, Ann. des Sciences Nat., xxvii, page 235) and others have remarked that they do not have 

 fertile crosses among themselves, but only when interbreeding with one of the primitive species 

 from which they had been derived. Mr. Bartlett {Proc. Zool. Soc, 1884, page 399, pi. xxxiv, xxxv) 

 has described a cross made in the Eegent's Park Zoological Gardens, which includes the races 

 of Zebra, Gayal, and Bison, and in which communication he gave descriptions of other hybrids 

 between mammals kept in a semi-domesticated state. Another instructive instance has been 

 recorded by Professor Kuhn, of Halle, respecting the interbreeding of the Gayal, Bos frontalie, 

 and some of the ordinary breeds of the domestic ox of Europe. A young bull and a cow Gayal 

 were received from Calcutta, June 18th, 1880 ; the bull paired readily with cows of every variety 



