Page:The New International Encyclopædia 1st ed. v. 10.djvu/416

* HYBRIDITY. .360 HYBRIDITY. geese (Anser cygnoidcs) arc fertile, as are hy- brids between the euiniiidii duck and the pintail duck. The crossed olVspring from the Indian liumpcd and conunon callli' inti-rliri-cd, Caton lias iivl)ridized llie Virginia deer with the Ceylon (leer and the Acapulcu dfcr, and says that the hybrids seem [ferfectly hfalthy and prolitir. The Indian dog and coyote arc said by Coucs to inter- breed, and on the upixr Missouri dogs have been seen having every appearance of lieing such hy- brids. Hybrids of hares and rabbits have con- tinued fruitful for generations; the same is true, says Ilertwig, of hybrids obtainc<l from the wild buck and the domesticated she goat. The Ameri- can bison is known to breed with the domestic cattle, and it sccnis to be a well-established fact that the hybrids are fertile. Kwart states that the Indian biitlalo and the American bison pro- duce fertile hybrids with the wild ox of Europe. Among fishes, fertile hybrids have been obtained from the carp and goldfish, and from the charr and the brook trout. Relative Strength ok Hereditary Influ- ences. The three following conclusions rest on an elaborate .scries of experiments by Standfuss with three European species of Saturnia moths: (1) The freshly hatched hybrid larva closely resembles that of the female parent, but with the process of growth a resemblance to that of the male parent gradually increases. (2) The extent of approximation toward the male parent depends on the relative phylogcnetic age of the two species, the older being able to transmit its properties, whether of stnicture or habit. l)etter than the younger. (3) reciprocal ))airing. the male is able to transmit the characters of the species in a higher degree than the female. This inlluence, however, is less regular and potent than that spoken of in the preceding paragraph. The result of crossinfi a piirciil spccici irith a local race, or irith an aberration of the same spe- cies, is as follows: ( 1 ) When the normal form of a species ( flrxind- art) is crossed with ii gradually formed local race of the same species, the result is :i series of intermediate forms. (2) On the other hand, when the normal form is crossed with a sporadic aberration, the result in many cases is that the issue divides itself sharply lietween the normal form and the sport, intermediate fonns being absent. Tn commenting on these results. Dr. F. . Dixey remarks: (1) The exix'riments afford fresh illus- trations of the manner in which the physiological isolation of an incipient species may be brought about. (2) They show that the statement of Focke as to the great variability of the offspring resulting from the crossing of a plant hybrid with one of the parent species holds good in the case of insects. (3) What Standfuss speaks of as the prepotency of the phylogcnetically older of the parent species is probably only another expression of the principle established by Dar- win that in many cases crossing causes reversion to a remote ancestor. (4) The general conclu- sion as to the prepotency of the male parent ac- cords so far with one resilt of Walton's investiga- tion of the late Sir E. ^lillais's breed of basset hounds. (H) The result of crossing a parent spe- cies with a gradually formed local race, though less in degree, is much the same in kind as that of crossing two distinct species. (6) The result of crossing the nonual form of a species with a sporadic aberration of the same s]x>cies appears to show that the latter stand biologically on an entirely dillercnt footing from the regularly de- velojx'd variety, even though it may indicate (as alleged by Eimcr and by .Jordan) the ilircc- tion in which variation for that species is pos- .siblc. With Standfuss's instances may be com- pared the well-known case of the 'otter sheep,' which similarly, when crossed with a sheep of ordinary breed, gave no true intermediates. (7) Certain experiments with alK'rrations of this kind, of which exact numerical records have been kept for .several generations, arc of special inter- est in connection with Galton's law of heredity. .See (ialton. atural Inheritance, p. 134 (Lon- don, 1889). In still later experiments by .Standfuss (1808) he shows that the crossing of two distinct sp<'cies gives rise to a zicischcnform, but not to a mitlcl- form. The latter may. however, exist as a tem- porary stage in larval growth. This depends on the following principles: ( I ) The freshly hatched larva closely resembles the female parent: (2) with the |)rocess of growth a resemblance to the male |)arcnt gradually increases: (3) the final extent of a])proxiination toward the male parent depends on the relative phylogenetic age of the two species, the oUler being able to transmit its properties, whether of structure or habit, better than the younger: (4) in reciprocal pair- ing the male is able to transmit the characters of the species in a higher degree than the female: (5) when the normal form of a species Klrund- art) is crossed with a gradually formed local race of the same species, the result is a scries of intermediate fornis: (Ti) when the normal forni is crossed with a sporadic aberration, the result in many cases is that the issue divides itself sharply between the normal form and the sport, intermediate forms being absent. Hence, according to Standfuss. adds Dixey. "the process of species formation must be gradual; for when two distinct s[x'cies are crossed the issue (iocs not split up into two parental forms, as in the case Avhcn one parent is a suddenly formed aberration. On the contrary, the behavior of the issue of two distinct species is very similar in kind to that of a species crossed with a local race or variety which is being gradually estab- lished by the accumulation of slight changes. It would seem, therefore, that although an aberra- tion or sport may be perpetuated by inheritance, it can never acquire distinct specific rank. No doTibt. however, it may. if selected, eventually replace the original form of the species." Dixey also makes the interesting suggestion that these sporadic color aberrations "seem to have many points of resemblance with the color varieties in domestic animals, such as the 'lemon and white' and 'tricolor' of the basset hounds, or the well-known tortoise-shell, tabby, and bl.ick of cats. The faet that these domestic varieties exist side by side in the same race, and even in the same litter, and that true intermediates are rare or absent, seems to suggest that tbev orig- inally appeared as sports, and that their per- Iietiation has been insured or favored by artificial select io7i." Zebra Hybrids. In his Penycuik Experiments (18!t0) Ewart states that he bred nine zebra hybrids by crossing mares of various sizes and breeds with a Burchell zebra stallion 'Matopo.'