Finally, from what we’ve got now seen of the relation which exists in many natural species and domesticated races, between the interval of the event of their characters and the style of their transmission-for example, the hanging reality of the early development of the horns within the reindeer, in which each sexes bear horns, compared with their much later growth in the opposite species by which the male alone bears horns-we might conclude that one, though not the only real cause of characters being completely inherited by one sex, is their improvement at a late age. That is definitely a hanging affirmation of my two laws of inheritance. Not only are the legal guidelines of inheritance extraordinarily complicated, however so are the causes which induce and govern variability. With the breeds of the Fowl the inheritance of various characters by one or each sexes, seems generally determined by the interval at which such characters are developed. In the sport breeds pugnacity is developed at a wonderfully early age, of which curious proofs could possibly be given; and this character is transmitted to both sexes, so that the hens, from their extreme pugnacity, are now usually exhibited in separate pens. The laced plumage of the Sebright bantam is identical in each sexes, and in the young chickens the wing-feathers are distinctly, though imperfectly laced.
On the other hand, in many of the breeds in which the two sexes resemble each other, the younger are colored in nearly the same manner as their dad and mom, and this renders it probable that their colours first appeared early in life. Summary and concluding remarks.-From the foregoing dialogue on the various legal guidelines of inheritance, we study that the characters of the parents usually, or even typically, tend to develop into developed in the offspring of the same intercourse, at the identical age, and periodically at the same season of the 12 months, wherein they first appeared in the mother and father. Thus in all the various breeds during which the grownup male differs drastically in color from the female, as well as from the wild mum or dad-species, he differs also from the younger male, so that the newly-acquired characters will need to have appeared at a quite late period of life. The male and female Peacock differ conspicuously from each other in almost every a part of their plumage, besides in the elegant head-crest, which is common to both sexes; and this is developed very early in life, long before the opposite ornaments, that are confined to the male.
The variations thus induced are preserved and accumulated by sexual selection, which is in itself a particularly complicated affair, relying, as it does, on the ardour in love, the courage, and the rivalry of the males, in addition to on the powers of notion, the style, and will of the feminine. The adult male can, however, be distinguished from the grownup female by the presence of spurs; and conformably with our rule, these don’t begin to be developed earlier than the age of six months, as I am assured by Mr. Bartlett, and even at this age, the two sexes can hardly be distinguished. Strictly analogous instances happen on the successive moults of sure male crustaceans. The sexes of canine don’t differ, besides that in sure breeds, especially in the Scotch deer-hound, the male is much larger and heavier than the feminine; and, as we shall see in a future chapter, the male goes on growing in size to an unusually late period of life, which, in accordance with rule, will account for his increased dimension being transmitted to his male offspring alone. Hence during the modification of a species, the successive adjustments might readily be transmitted in alternative ways; some to at least one intercourse, and some to both; some to the offspring at one age, and some to the offspring at all ages.
Symptoms could occur more often. Spangled Hamburgs, nevertheless, offer a partial exception; for the two sexes, though not quite alike, resemble one another more closely than do the sexes of the aboriginal guardian-species; yet they acquire their characteristic plumage late in life, for the chickens are distinctly pencilled. The horns of sheep, goats, and cattle, which are effectively developed in both sexes, although not quite equal in dimension, might be felt, or even seen, at birth or soon afterwards. In the varied home breeds of sheep, goats, and cattle, the males differ from their respective females in the form or growth of their horns, forehead, mane, dewlap, tail, and hump on the shoulders; and these peculiarities, in accordance with our rule, will not be absolutely developed till a quite late interval of life. There’s a breed of pigeons in which the males alone are streaked with black, and the streaks might be detected even within the nestlings; however they turn into more conspicuous at every successive moult, in order that this case partly opposes and partly helps the rule. But this rule as before remarked, does not look like nearly so normal as the converse one, specifically, that characters which seem late in life in one sex are transmitted solely to the identical sex.