HYBRIDS— FERTILITY OF. 269 



The next set of experiments consisted in crossing hybrids with pure breeds. 

 On Nov. I2th, 1884, when the struans were only 21 months old their milt was 

 used in order to fertilize some Loohleven trout eggs, and the mortality during 

 incubation was 63 per cent. It has been previously pointed out that male 

 salmonoids are mostly more advanced for procreative purposes than females are. 

 There were many deformities and albinos among these offspring. This cross was 

 again tried, Dec. 5th, 1885, when the mortality was about 78 per cent, and many 

 of the oflFspring were albinos. 



It would appear from the foregoing, it is probable when crossing two races of 

 which the male belongs to the larger breed, that fertilization of the ova may be 

 prevented owing to the size of the micropyle being insuificient to freely admit the 

 entrance of the spermatozoa. 



That some physiological cause must be in existence which occasions deformities 

 or monstrosities, and many eggs are unfertilized. Also that in such crosses the 

 offspring are very weak. 



The colours in hybrid (zebra) offspring crossed by a race of pure Lochleven 

 trout have not reverted to those of either of the original parents, but appear to be 

 forming those of a very distinct and separate kind. 



It will now be necessary to notice the number of days the eggs of hybrids 

 require for incubation and to compare them with what takes place in those 

 of the parent species kept distinct but under similar conditions.* December, 

 1881, Lochleven trout eggs milted from salmon hatched in 75 days ; and in 

 December, 1884, salmon eggs milted from Lochleven trout took *?& days. December, 

 1884, some salmon ova were milted from salmon and incubated under the same 

 conditions as the hybrids, and they hatched in 79 days. In fact the hybrids, 

 whichever species was the male parent, hatched four or five days before the home- 

 bred fish. 



On each occasion of the milt or eggs from a Howietoun raised grilse being 

 crossed by Lochleven trout they took 78 days in incubating. 



It must be self-evident that should any cause occasion salmon or trout or char 

 eggs to be hatched prior to the normal number of days of incubation having 

 elapsed, such cannot conduce to the strength of the offspring. We know this 

 may be effected by elevating temperature (see pp. 35, 36 ante), and that weak 

 alevins are the result. But when this is done by other means, as employing the 

 male of a species (admittedly healthy) wherein incubation requires a moderate 

 number of days to fertilize eggs from a healthy female of another species which 

 requires a much larger number of days, and the period of incubation becomes 

 reduced, it would seem to be a most probable result that certain pathological 

 conditions would be set up in the offspring, such as were perceived in some of the 

 hybrids already referred to. 



The American char, Salmo fontinalis, at from 43° to 43'5° Fahr. requires 

 from 79 to 81 days for incubation, and crossing a female with a young salmon the 

 period required was 83 days, and in two other instances where Lochleven trout were 

 the male element 84 and 85 days : when the British char was similarly employed 

 86 days. But when we examine the period required for the eggs of hybrids inter- 

 bred among themselves we see another alteration, almost a regular diminution in 

 the number of days, as among the struans they varied from 79 to 83 days, but 

 crossing these hybrids with British char or Lochleven trout only 77 days were 

 required. 



Respecting the number of par bandsf on the sides of young hybrid Salmonidce 



* During the winter of 1886-7, at Howietoun, with the temperature of the water kept at 43° or 

 43-5° Fahr., the following number of days was found necessary for incubating the eggs of the 

 following fish: — Salmon, pure, hatched at Howietoun, laid down in November, 81 days: sea trout, 

 S. truUa, laid down in November, 81 days : burn trout, Salmo fario, not above 3 years of age, laid 

 down November 11th, 79 days in one lot, 74 in another : Loohleven trout, pure 10-year-old fish, 

 laid down in November, 76 days : Loclileveu, 8-year-old, milted in November by young burn 

 trout, 75 days : Loohleven, B-year-old, milted in November by burn trout, 76 days : American 

 char, S. fontinalis, laid down in November, from 79 to 81 days. 



t The number of bands appears to be in some way modified by locality; if the young at 

 Howietoun are reared in water coming from Loch Coulter they have about 17, if from the small 



